Regulation and Function of AQP4 in the Central Nervous System

The glymphatic system: a new perspective on brain diseases

The glymphatic system is a brain-wide perivascular pathway driven by aquaporin-4 on the endfeet of astrocytes, which can deliver nutrients and active substances to the brain parenchyma through periarterial cerebrospinal fluid (CSF) influx pathway and remove metabolic wastes through perivenous clearance routes. This paper summarizes the composition, overall fluid flow, solute transport, related diseases, affecting factors, and preclinical research methods of the glymphatic system. In doing so, we aim to provide direction and reference for more relevant researchers in the future.

AQP4 mitigates chronic neuropathic pain-induced cognitive impairment in mice

Neuropathic pain is a risk factor for cognitive defects. The ubiquitous expression of AQP4 in astrocytes throughout the central nervous system is altered in the neurodegenerative disease. However, the exact role of AQP4 in cognitive impairment induced by chronic neuropathic pain remains unclear. In this study, we discovered that AQP4 protein and mRNA expression decreased time-dependently in the model of chronic neuropathic pain-induced cognitive disorder. AQP4 overexpression recovered mice from cognitive impairment. Furthermore, the concentration of Aβ1-42 in the serum and hippocampus reduced in mice with AQP4 overexpression adeno-associated virus injection. In conclusion, AQP4 in astrocytes is important in mitigating cognitive impairment caused by chronic neuropathic pain.

Aquaporins Display a Diversity in their Substrates

Aquaporins constitute a family of transmembrane proteins that function to transport water and other small solutes across the cell membrane. Aquaporins family members are found in diverse life forms. Aquaporins share the common structural fold consisting of six transmembrane alpha helices with a central water-transporting channel. Four such monomers assemble together to form tetramers as their biological unit. Initially, aquaporins were discovered as water-transporting channels, but several studies supported their involvement in mediating the facilitated diffusion of different solutes. The so-called water channel is able to transport a variety of substrates ranging from a neutral molecule to a charged molecule or a small molecule to a bulky molecule or even a gas molecule. This article gives an overview of a diverse range of substrates conducted by aquaporin family members. Prime focus is on human aquaporins where aquaporins show a wide tissue distribution and substrate specificity leading to various physiological functions. This review also highlights the structural mechanisms leading to the transport of water and glycerol. More research is needed to understand how one common fold enables the aquaporins to transport an array of solutes.

Aquaporins in Nervous System

Aquaporins (AQPs) mediate water flux between the four distinct water compartments in the central nervous system (CNS). In the present chapter, we mainly focus on the expression and function of the nine AQPs expressed in the CNS, which include five members of aquaporin subfamily: AQP1, AQP4, AQP5, AQP6, and AQP8; three members of aquaglyceroporin subfamily: AQP3, AQP7, and AQP9; and one member of superaquaporin subfamily: AQP11. In addition, AQP1, AQP2, and AQP4 expressed in the peripheral nervous system are also reviewed. AQP4, the predominant water channel in the CNS, is involved both in the astrocyte swelling of cytotoxic edema and the resolution of vasogenic edema and is of pivotal importance in the pathology of brain disorders such as neuromyelitis optica, brain tumors, and neurodegenerative disorders. Moreover, AQP4 has been demonstrated as a functional regulator of recently discovered glymphatic system that is a main contributor to clearance of toxic macromolecule from the brain. Other AQPs are also involved in a variety of important physiological and pathological process in the brain. It has been suggested that AQPs could represent an important target in treatment of brain disorders like cerebral edema. Future investigations are necessary to elucidate the pathological significance of AQPs in the CNS.

Acutely increased aquaporin-4 exhibits more potent protective effects in the cortex against single and repeated isoflurane-induced neurotoxicity in the developing rat brain

Damage to hippocampus, cerebellum, and cortex associated with cognitive functions due to anesthetic-induced toxicity early in life may cause cognitive decline later. Aquaporin 4 (AQP4), a key protein in waste clearance pathway of brain, is involved in synaptic plasticity and neurocognition. We investigated the effects of single and repeated isoflurane (Iso) anesthesia on AQP4 levels and brain damage. Postnatal-day (P)7 Wistar albino rats were randomly assigned to Iso or Control (C) groups. For single-exposure, pups were exposed to 1.5% Iso in 30% oxygenated-air for 3-h at P7 (Iso₁). For repeated-exposure, pups were exposed to Iso for 3 days, 3-h each day, at 1-day intervals (P7 + 9+11) starting at P7 (Iso₃). C₁ and C₃ groups received only 30% oxygenated-air. Based on HE-staining and immunoblotting (Bax/Bcl-2, cleaved-caspase3 and PARP1) analyses, Iso exposures caused a higher degree of apoptosis in hippocampus. Anesthesia increased 4HNE, oxidative stress marker; the highest ROS accumulation was determined in cerebellum. Increased inflammation (TNF-α, NF-κB) was detected. Multiple Iso-exposures caused more significant damage than single exposure. Moreover, 4HNE and TNF-α contributed synergistically to Iso-induced neurotoxicity. After anesthesia, higher expression of AQP4 was detected in cortex than hippocampus and cerebellum. There was an inverse correlation between increased AQP4 levels and apoptosis/ROS/inflammation. Correlation analysis indicated that AQP4 had a more substantial protective profile against oxidative stress than apoptosis. Remarkably, acutely increased AQP4 against Iso exhibited a more potent neuroprotective effect in cortex, especially frontal cortex. These findings promote further research to understand better the mechanisms underlying anesthesia-induced toxicity in the developing brain.

Emerging role of LINC00461 in cancer

LINC00461 is located in the intergenic region between the protein-coding genes MEF2C and TMEM161B. LINC00461 upregulation was associated with the risk of 13 tumors and was strongly associated with clinicopathologic features and poor prognosis in 11 tumors. LINC00461 is involved in resistance to four anticancer drugs, including sunitinib for renal cell carcinoma, cisplatin for head and neck squamous cell carcinoma and rectal cancer, temozolomide for glioma, and docetaxel for breast cancer. LINC00461 can sponge 18 miRNAs to form a complex ceRNA network that regulates the expression of a large number of downstream genes. LINC00461 is involved in the MAPK/ERK signaling pathway and PI3K/AKT signaling pathway, thereby promoting tumorigenesis. Notably, knockdown of LINC00461 in exosomes antagonizes tumor cell proliferation in multiple myeloma. This article summarizes the diagnostic, prognostic, and therapeutic value of LINC00461 in various tumors, and systematically describes the ceRNA network and signaling pathways associated with LINC00461, providing potential directions for future LINC00461 research.

Elevated Intracranial Pressure in Cryptococcal Meningoencephalitis: Examining Old, New, and Promising Drug Therapies

Despite the availability of effective antifungal therapy, cryptococcal meningoencephalitis (CM) remains associated with elevated mortality. The spectrum of symptoms associated with the central nervous system (CNS) cryptococcosis is directly caused by the high fungal burden in the subarachnoid space and the peri-endothelial space of the CNS vasculature, which results in intracranial hypertension (ICH). Management of intracranial pressure (ICP) through aggressive drainage of cerebrospinal fluid by lumbar puncture is associated with increased survival. Unfortunately, these procedures are invasive and require specialized skills and supplies that are not readily available in resource-limited settings that carry the highest burden of CM. The institution of pharmacologic therapies to reduce the production or increase the resorption of cerebrospinal fluid would likely improve clinical outcomes associated with ICH in patients with CM. Here, we discuss the potential role of multiple pharmacologic drug classes such as diuretics, corticosteroids, and antiepileptic agents used to decrease ICP in various neurological conditions as potential future therapies for CM.

Creatine transporter-deficient rat model shows motor dysfunction, cerebellar alterations, and muscle creatine deficiency without muscle atrophy

Creatine (Cr) is a nitrogenous organic acid and plays roles such as fast phosphate energy buffer to replenish ATP, osmolyte, antioxidant, neuromodulator, and as a compound with anabolic and ergogenic properties in muscle. Cr is taken from the diet or endogenously synthetized by the enzymes arginine:glycine amidinotransferase and guanidinoacetate methyltransferase, and specifically taken up by the transporter SLC6A8. Loss-of-function mutations in the genes encoding for the enzymes or the transporter cause creatine deficiency syndromes (CDS). CDS are characterized by brain Cr deficiency, intellectual disability with severe speech delay, behavioral troubles, epilepsy, and motor dysfunction. Among CDS, the X-linked Cr transporter deficiency (CTD) is the most prevalent with no efficient treatment so far. Different animal models of CTD show reduced brain Cr levels, cognitive deficiencies, and together they cover other traits similar to those of patients. However, motor function was poorly explored in CTD models, and some controversies in the phenotype exist in comparison with CTD patients. Our recently described Slc6a8^Y389C knock-in rat model of CTD showed mild impaired motor function, morphological alterations in cerebellum, reduced muscular mass, Cr deficiency, and increased guanidinoacetate content in muscle, although no consistent signs of muscle atrophy. Our results indicate that such motor dysfunction co-occurred with both nervous and muscle dysfunctions, suggesting that muscle strength and performance as well as neuronal connectivity might be affected by this Cr deficiency in muscle and brain.

Aquaporin-4 and Cognitive Disorders

Aquaporin-4 (AQP4) is the most abundantly expressed aquaporin in the central nervous system (CNS) and is an integral part of the glymphatic system that cannot be ignored. The CNS has the glymphatic system instead of the conventional lymphatic system. The glymphatic system plays an essential role in the pathophysiological processes of many cognitive disorders. AQP4 shows noteworthy changes in various cognitive disorders and is part of the pathogenesis of these diseases. For this reason, AQP4 has attracted attention as a potential and promising target for regulating and even reversing cognitive dysfunction. This review will summarize the role of AQP4 in the pathophysiological processes of several cognitive disorders as reported in recent studies.

The role of aquaporin 4 (AQP4) in spinal cord injury

Aquaporin-4 (AQP-4) is an aquaporin composed of six helical transmembrane domains and two highly conserved ASN-pro-ALA (NPA) motifs. It is strongly expressed in rodent and human spinal cord tissues and plays a key role in the pathological process after SCI. After SCI, edema, glial scarring, and inflammation can accelerate the progression of injury and lead to deterioration of function. Many studies have reported that AQP-4 plays an important role in SCI. In particular, it plays an important role in secondary pathological processes (spinal cord edema, glial scar formation, and inflammatory response) after SCI. Loss of AQP-4 has been associated with reduced spinal edema and improved prognosis after SCI in mice. In addition, downregulation of AQP-4 reduces glial scar formation and the inflammatory response after SCI. There is a consensus from numerous studies that AQP-4 may be a potential target for SCI therapy, which guides the ongoing investigation for molecular therapy of SCI. Here, we review the structure of AQP-4, its expression in normal and damaged spinal cord, and its role in SCI, as well as discuss the theoretical basis for the treatment of SCI.

Molecular mechanisms of brain water transport

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.

Carvacrol as a Potential Neuroprotective Agent for Neurological Diseases: A Systematic Review Article

BACKGROUND & OBJECTIVE

Neurological diseases are becoming a significant problem worldwide, with the elderly at a higher risk of being affected. Several researchers have investigated the neuroprotective effects of Carvacrol (CAR) (5-isopropyl-2-methyl phenol). This review systematically surveys the existing literature on the impact of CAR when used as a neuroprotective agent in neurological diseases.

METHOD

The systematic review involved English articles published in the last ten years obtained from PubMed, Google Scholar, and Scopus databases. The following descriptors were used to search the literature: "Carvacrol" [Title] AND "neuroprotective (neuroprotection)" [Title] OR "stroke, traumatic brain injury, Alzheimer's disease, Parkinson's disease, seizure, epilepsy [Title].

RESULTS

A total of 208 articles were retrieved during the search process, but only 20 studies met the eligibility criteria and were included for review. A total of 20 articles were identified, in which the efficacy of CAR was described in experimental models of stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, , epilepsy, and seizure, through motor deficits improvements in neurochemical activity, especially antioxidant systems, reducing inflammation, oxidative stress and apoptosis as well as inhibition of TRPC1 and TRPM7.

CONCLUSION

The data presented in this study support the beneficial impact of CAR on behavioural and neurochemical deficits. CAR benefits accrue because of its anti-apoptotic, antioxidant, and anti-inflammatory properties. Therefore, CAR has emerged as an alternative treatment for neurological disorders based on its properties.

Parameters Indicating Development of Influenza-Associated Acute Necrotizing Encephalopathy: Experiences from a Single Center

Background

Influenza-associated acute necrotizing encephalopathy (IANE) can be lethal and disabling and have a sudden onset and deteriorate rapidly but lacks early diagnostic indicators. We aimed to examine the early clinical diagnostic indicators in children with IANE.

Material/Methods

Acute influenza patients were grouped according to their clinical manifestations: flu alone (FA), flu with febrile seizure (FS), influenza-associated encephalopathy (IAE), and IANE. The clinical features, biomarkers, neuroelectrophysiological results, and neuroimaging examination results were compared.

Results

A total of 31 patients were included (FA (n=4), FS (n=8), IAE (n=14), and IANE (n=5)). The IANE group, whose mean age was 3.7 years, was more likely to show rapid-onset seizure, acute disturbance of consciousness (ADOC), Babinski’s sign, and death/sequela. More patients in the IANE group required tracheal intubation mechanical ventilation and received intravenous immunoglobulins (IVIG) and glucocorticoids. The alanine aminotransferase (ALT), aspartate transaminase (AST), and lactate dehydrogenase (LDH) levels in the IANE group were significantly higher than in the FS and IAE groups. The aquaporin-4 (AQP-4) antibody and malondialdehyde (MDA) levels in the serum and cerebrospinal fluid (CSF) were notably higher in IANE patients in the acute stage compared with FS and IAE patients. All patients in the IANE group had positive neuroimaging findings.

Conclusions

Early clinical warning factors for IANE include rapid-onset seizures in patients under 4 years of age, ADOC, and pathological signs. Increased AQP-4 antibodies and MDA levels in CSF might contribute to early diagnosis. Early magnetic resonance venography (MRV) and susceptibility-weighted imaging (SWI) sequences, or thrombelastography to identify deep vein thrombosis, might indicate clinical deterioration.

Differentially regulated pools of aquaporin-4 (AQP4) proteins in the cerebral cortex revealed by biochemical fractionation analyses

Aquaporin-4 (AQP4) is a predominant water channel in the central nervous system. It regulates water movement in the brain and has been suggested to play critical roles in various pathological conditions. However, the molecular mechanisms underlying its regulation are not yet well understood. In this study, we biochemically characterized AQP4 in the brain using acute cortical brain slices prepared from mice. Using biochemical fractionation, we found that AQP4 is enriched in the detergent-resistant membrane (DRM) fraction that is not soluble in 1% Triton X-100. In contrast, β-dystroglycan and syntrophin, which are part of the dystrophin complex in the brain, primarily reside in the non-DRM fraction. DRM enrichment of AQP4 is insensitive to cholesterol depletion, suggesting that it is not tightly associated with lipid rafts. Furthermore, AQP4 in the DRM fraction is more enriched in the M23 isoform than in the non-DRM fraction. Finally, by employing oxygen-glucose deprivation (OGD), an in vitro model of ischemia, we examined the molecular changes of AQP4. Under OGD conditions, a reduction in AQP4 in the DRM fraction was observed before the total AQP4 protein level dropped. Our data therefore highlight the characteristics of two pools of AQP4 that are distinctly regulated under ischemic conditions.

Hydrogen sulfide attenuates hyperhomocysteinemia-induced blood-brain barrier permeability by inhibiting MMP-9

Backgroud: Hyperhomocysteinemia (HHcy) is implicated in various neurovascular disorders including vascular dementia, subarachnoid hemorrhage and stroke. Elevated homocysteine (Hcy) levels are associated with increased oxidative stress and compromised blood-brain barrier (BBB) integrity. Hydrogen sulfide (H₂S) has recently emerged as potent neuroprotective molecule in various neurological conditions including those associated with HHcy. The present study evaluates the protective effect of sodium hydrogen sulfide (NaHS; a source of H₂S) on HHcy-induced BBB dysfunction and underpin molecular mechanisms.Materials and methods: Supplementation of NaHS restored the increased BBB permeability in the cortex and hippocampus of HHcy animals assessed in terms of diffused sodium fluorescein and Evans blue tracer dyes in the brain. Activity of matrix metalloproteinases (MMPs) assessed by gelatinase activity and in situ gelatinase assay was restored to the normal in the cortex and hippocampus of HHcy animals supplemented with NaHS.Results: Application of gelatin zymography revealed that specifically MMP-9 activity was increased in the cortex and hippocampus of HHcy animals, which was inhibited by NaHS supplementation. Real-time RT-PCR analysis showed that NaHS administration also decreased mRNA expression of MMP-9 in the hippocampus of HHcy animals. NaHS supplementation was further observed to reduce water retention in the brain regions of Hcy treated animals.Conclusion: Taken together, these findings suggest that NaHS supplementation ameliorates HHcy-induced BBB permeability and brain edema by inhibiting the mRNA expression and activity of MMP-9. Therefore, H₂S and H₂S releasing drugs may be used as a novel therapeutic approach to treat HHcy-associated neurovascular disorders.

Poldip2 mediates blood-brain barrier disruption and cerebral edema by inducing AQP4 polarity loss in mouse bacterial meningitis model

Background

Specific highly polarized aquaporin‐4 (AQP4) expression is reported to play a crucial role in blood‐brain barrier (BBB) integrity and brain water transport balance. The upregulation of polymerase δ‐interacting protein 2 (Poldip2) was involved in aggravating BBB disruption following ischemic stroke. This study aimed to investigate whether Poldip2‐mediated BBB disruption and cerebral edema formation in mouse bacterial meningitis (BM) model occur via induction of AQP4 polarity loss.

Methods and Results

Mouse BM model was induced by injecting mice with group B hemolytic streptococci via posterior cistern. Recombinant human Poldip2 (rh‐Poldip2) was administered intranasally at 1 hour after BM induction. Small interfering ribonucleic acid (siRNA) targeting Poldip2 was administered by intracerebroventricular (i.c.v) injection at 48 hours before BM induction. A specific inhibitor of matrix metalloproteinases (MMPs), UK383367, was administered intravenously at 0.5 hour before BM induction. Western blotting, immunofluorescence staining, quantitative real‐time PCR, neurobehavioral test, brain water content test, Evans blue (EB) permeability assay, transmission electron microscopy (TEM), and gelatin zymography were carried out. The results showed that Poldip2 was upregulated and AQP4 polarity was lost in mouse BM model. Both Poldip2 siRNA and UK383367 improved neurobehavioral outcomes, alleviated brain edema, preserved the integrity of BBB, and relieved the loss of AQP4 polarity in BM model. Rh‐Poldip2 upregulated the expression of MMPs and glial fibrillary acidic protein (GFAP) and downregulated the expression of β‐dystroglycan (β‐DG), zonula occludens‐1 (ZO‐1), occludin, and claudin‐5; whereas Poldip2 siRNA downregulated the expression of MMPs and GFAP, and upregulated β‐DG, ZO‐1, occludin, and claudin‐5. Similarly, UK383367 downregulated the expression of GFAP and upregulated the expression of β‐DG, ZO‐1, occludin, and claudin‐5.

Conclusion

Poldip2 inhibition alleviated brain edema and preserved the integrity of BBB partially by relieving the loss of AQP4 polarity via MMPs/β‐DG pathway.

Nrf2 Deficiency Exacerbates Cognitive Impairment and Reactive Microgliosis in a Lipopolysaccharide-Induced Neuroinflammatory Mouse Model

The transcription factor Nrf2 is a central regulator of anti-inflammatory and antioxidant mechanisms that contribute to the development and progression of various neurological disorders. Although the direct and indirect Nrf2 regulatory roles on inflammation have been reviewed in recent years, the in vivo evidence of Nrf2 function on lipopolysaccharide (LPS)-induced cognitive decline and characteristic alterations of reactive microglia and astrocytes remains incomplete. During the 3-5 days after LPS or saline injection, 5-6-month-old wildtype (WT) and Nrf2^-/- C57BL/6 mice were subjected to the novel object recognition task. Immunohistochemistry staining was employed for analyses of brain cells. The Nrf2^-/- mice displayed exacerbated LPS-induced cognition impairment (28.1 ± 9.6% in the discrimination index of the novel object recognition task), enhanced hippocampal reactive microgliosis and astrogliosis, and an increased expression level of the water channel transmembrane protein aquaporin 4 when compared with WT controls. In addition, similar overt effects of Nrf2 deficiency on LPS-induced characteristic alterations of brain cells were observed in the cortex and striatum regions of mice. In summary, this transgenic loss-of-function study provides direct in vivo evidence that highlights the functional importance of Nrf2 activation in regulating LPS-induced cognitive alteration, glial responses, and aquaporin 4 expression. This finding provides a better understanding of the complex nature of Nrf2 signaling and neuroprotection.

Evolutionary Adaptation of Aquaporin-4 in Yak (Bos grunniens) Brain to High-Altitude Hypoxia of Qinghai-Tibetan Plateau

Background: In high-altitude animals, brain cell resilience against hypoxia stress is one critical evolutionary step that has promoted individual survival and species adaptation to the environment. Aquaporin-4 (AQP4) is implicated in a number of physiopathological processes, particularly in the development of brain edema, and other functions such as the regulation of extracellular space volume, potassium buffering, waste clearance, and calcium signaling. Still, the role of AQP4 in the adaptation to high-altitude hypoxia remains unknown. The yak (Bos grunniens) is the only large mammal that is currently known to have adapted to the high-altitude hypoxic environment of the Qinghai-Tibet Plateau (>4000 m above sea level). Methods: In this study, we cloned the complementary DNA (cDNA) for yak AQP4 and analyzed structural differences of AQP4 between yak and cattle. We used reverse transcription quantitative polymerase chain reaction and western blot to investigate whether the expression of AQP4 mRNA and protein was different in brain of yak and cattle. In addition, immunohistochemistry was use to analyze the localization and expression of AQP4 in brain of yak and cattle. Results: Immunohistochemical results have shown that AQP4 is expressed in many regions of the yak brain, and both protein and messenger RNA (mRNA) levels are significantly lower than those of low-altitude cattle (Bos taurus). Phylogenetic analysis revealed that yak AQP4 is evolutionarily conserved. Interestingly, a substitution of Ala (cattle) to Ser in position 82, and eight additional amino acid residues composing an α-helix region are present in yak AQP4 protein. These sequence modifications potentially modulate the function of AQP4 in distinct environments. Conclusions: Our findings suggest that AQP4 may have an important role in the resistance to cerebral edema through low expression and maintenance of normal physiological function in the yak brain.

Regulation of AQP4 in the Central Nervous System

Aquaporin-4 (AQP4) is the main water channel protein expressed in the central nervous system (CNS). AQP4 is densely expressed in astrocyte end-feet, and is an important factor in CNS water and potassium homeostasis. Changes in AQP4 activity and expression have been implicated in several CNS disorders, including (but not limited to) epilepsy, edema, stroke, and glioblastoma. For this reason, many studies have been done to understand the various ways in which AQP4 is regulated endogenously, and could be regulated pharmaceutically. In particular, four regulatory methods have been thoroughly studied; regulation of gene expression via microRNAs, regulation of AQP4 channel gating/trafficking via phosphorylation, regulation of water permeability using heavy metal ions, and regulation of water permeability using small molecule inhibitors. A major challenge when studying AQP4 regulation is inter-method variability. A compound or phosphorylation which shows an inhibitory effect in vitro may show no effect in a different in vitro method, or even show an increase in AQP4 expression in vivo. Although a large amount of variability exists between in vitro methods, some microRNAs, heavy metal ions, and two small molecule inhibitors, acetazolamide and TGN-020, have shown promise in the field of AQP4 regulation.

Isoflurane exposure in infant rats acutely increases aquaporin 4 and does not cause neurocognitive impairment

Isoflurane is commonly used in pediatric population, but its mechanism of action in cognition is unclear. Aquaporin 4 (AQP4) regulates water content in blood, brain, and cerebrospinal fluid. Various studies have provided evidence for the role of AQP4 in synaptic plasticity and neurocognition. In this study, we aimed to determine whether a prolonged exposure to isoflurane in infant rats is associated with cognition and what effect this exposure has on AQP4 expression. Ten-day-old [postnatal day (P) 10] Wistar albino rats were randomly allocated to isoflurane group (n = 32; 1.5% isoflurane in 50% oxygen for 6 hours) or control group (n = 32; only 50% oxygen for 6 hours). Acute (P11) and long-term (P33) effects of 6-hour anesthetic isoflurane exposure on AQP4 expression were analyzed in whole brains of P11 and P33 rats by RT-qPCR and Western blot. Spatial learning and memory were assessed on P28 to P33 days by Morris Water Maze (MWM) test. The analysis revealed that isoflurane increased acutely both mRNA (~4.5 fold) and protein (~90%) levels of AQP4 in P11 rats compared with control group. The increasing levels of AQP4 in P11 were not observed in P33 rats. Also, no statistically significant change between isoflurane and control groups was observed in the latency to find the platform during MWM training and probe trial. Our results indicate that a single exposure to isoflurane anesthesia does not influence cognition in infant rats. In this case, acutely increased AQP4 after isoflurane anesthesia may have a protective role in neurocognition.

Astroglial endfeet exhibit distinct Ca2+ signals during hypoosmotic conditions

Astrocytic endfeet cover the brain surface and form a sheath around the cerebral vasculature. An emerging concept is that endfeet control blood-brain water transport and drainage of interstitial fluid and waste along paravascular pathways. Little is known about the signaling mechanisms that regulate endfoot volume and hence the width of these drainage pathways. Here, we used the genetically encoded fluorescent Ca²⁺ indicator GCaMP6f to study Ca²⁺ signaling within astrocytic somata, processes, and endfeet in response to an osmotic challenge known to induce cell swelling. Acute cortical slices were subjected to artificial cerebrospinal fluid with 20% reduction in osmolarity while GCaMP6f fluorescence was imaged with two-photon microscopy. Ca²⁺ signals induced by hypoosmotic conditions were observed in all astrocytic compartments except the soma. The Ca²⁺ response was most prominent in subpial and perivascular endfeet and included spikes with single peaks, plateau-type elevations, and rapid oscillations, the latter restricted to subpial endfeet. Genetic removal of the type 2 inositol 1,4,5-triphosphate receptor (IP3R2) severely suppressed the Ca²⁺ responses in endfeet but failed to affect brain water accumulation in vivo after water intoxication. Furthermore, the increase in endfoot Ca²⁺ spike rate during hypoosmotic conditions was attenuated in mutant mice lacking the aquaporin-4 anchoring molecule dystrophin and after blockage of transient receptor potential vanilloid 4 channels. We conclude that the characteristics and underpinning of Ca²⁺ responses to hypoosmotic stress differ within the astrocytic territory and that IP3R2 is essential for the Ca²⁺ signals only in subpial and perivascular endfeet.

Aquaporin-4 expression dynamically varies after acute spinal cord injury-induced disruption of blood spinal cord barrier in rats

The blood-spinal cord barrier (BSCB) changes badly after spinal cord injury (SCI), and it is an important pathophysiological basis of SCI secondary damage. Aquaporin-4 (AQP4), one of the transmembrane proteins in spinal cord, has been shown to be closely related to the development of the BSCB and edema. We established a SCI model in rats using a free-falling weight drop device to subsequently investigate AQP4 expression. AQP4 messenger RNA (mRNA) and protein expression and immunoreactivity were detected in spinal cord tissue using reverse transcription-real-time quantitative polymerase chain reaction (RT-qPCR), immunohistochemistry and Western blot analysis. We found the water content and edema of the spinal cord were significantly higher than the control group after SCI, which was related to the growth of BSCB permeability; both reached their peak on the third day after injury. One, 3, 5, 7 days after injury, the immune response and protein expression in the model group increased from 1 to 3 days, with a plateau period from 3 to 5 days and a decline from 5 to 7 days, showing a significant difference compared with the sham group at each time point (P < 0.05), while the RT-qPCR results showed a decline of mRNA just after 3 days. In conclusion, after SCI, the water content of the spinal cord and the BSCB permeability increases, together with the excessive expression of AQP4, which reached a peak on the third day. AQP4 expression is closely relative to the permeability of BSCB and the water content of the spinal cord.

Astrogliogenesis in human fetal brain: complex spatiotemporal immunoreactivity patterns of GFAP, S100, AQP4 and YKL-40

The astroglial lineage consists of heterogeneous cells instrumental for normal brain development, function and repair. Unfortunately, this heterogeneity complicates research in the field, which suffers from lack of truly specific and sensitive astroglial markers. Nevertheless, single astroglial markers are often used to describe astrocytes in different settings. We therefore investigated and compared spatiotemporal patterns of immunoreactivity in developing human brain from 12 to 21 weeks post conception and publicly available RNA expression data for four established and potential astroglial markers - GFAP, S100, AQP4 and YKL-40. In the hippocampal region, we also screened for C3, a complement component highly expressed in A1-reactive astrocytes. We found diverging partly overlapping patterns of the established astroglial markers GFAP, S100 and AQP4, confirming that none of these markers can fully describe and discriminate different developmental forms and subpopulations of astrocytes in human developing brain, although AQP4 seems to be the most sensitive and specific marker for the astroglial lineage at midgestation. AQP4 characterizes a brain-wide water transport system in cerebral cortex with regional differences in immunoreactivity at midgestation. AQP4 distinguishes a vast proportion of astrocytes and subpopulations of radial glial cells destined for the astroglial lineage, including astrocytes determined for the future glia limitans and apical truncated radial glial cells in ganglionic eminences, devoid of GFAP and S100. YKL-40 and C3d, previously found in reactive astrocytes, stain different subpopulations of astrocytes/astroglial progenitors in developing hippocampus at midgestation and may characterize specific subpopulations of 'developmental astrocytes'. Our results clearly reflect that lack of pan-astrocytic markers necessitates the consideration of time, region, context and aim when choosing appropriate astroglial markers.

Aquaporin water channels: New perspectives on the potential role in inflammation

Aquaporins (AQPs) are a family of membrane water channel proteins that osmotically modulate water fluid homeostasis in several tissues; some of them also transport small solutes such as glycerol. At the cellular level, the AQPs regulate not only cell migration and transepithelial fluid transport across membranes, but also common events that are crucial for the inflammatory response. Emerging data reveal a new function of AQPs in the inflammatory process, as demonstrated by their dysregulation in a wide range of inflammatory diseases including edematous states, cancer, obesity, wound healing and several autoimmune diseases. This chapter summarizes the discoveries made so far about the structure and functions of the AQPs and provides updated information on the underlying mechanisms of AQPs in several human inflammatory diseases. The discovery of new functions for AQPs opens new vistas offering promise for the discovery of mechanisms and therapeutic opportunities in inflammatory disorders.

Normobaric oxygen inhibits AQP4 and NHE1 expression in experimental focal ischemic stroke

The aim of the present study was to determine the effect of 60% normobaric oxygen (NBO) on neurological function, brain edema and the expression of hypoxia-inducible factor-1α (HIF-1α), aquaporin 4 (AQP4) and Na⁺/H⁺ exchanger 1 (NHE1) in a rat model of cerebral ischemia-reperfusion injury. Male Sprague-Dawley rats underwent transient focal cerebral ischemia via right middle cerebral artery occlusion (MCAO) for 120 min followed by 48 h of reperfusion. The rats were exposed to NBO at 60 and 100% or no treatment during reperfusion for 48 h. Neurological impairment score (NIS) was evaluated prior to the sacrifice of all rats. Hematoxylin-eosin staining was performed after 48 h of reperfusion with NBO treatment. The infarct volume and brain water content (BWC) were determined to assess brain ischemic injury at 24 and 48 h. The levels of HIF-1α, AQP4 and NHE1 expression in brain tissue samples were determined by western blotting and reverse transcription-quantitative polymerase chain reaction analysis. During reperfusion, the protein and mRNA expression of HIF-1α, AQP4 and NHE1 increased over time (up to 48 h). Exposure to 60 and 100% NBO during reperfusion following MCAO improved NIS, and alleviated BWC and infarct volume after 24 and 48 h, with further improvements in the 100% NBO group, compared with 60%. Additionally, the molecular mechanisms involved in the effects of NBO may be associated with reduced AQP4 and NHE1 expression and increased HIF-1α expression. However, 60% NBO therapy during reperfusion following an acute ischemic stroke did not achieve the same effects as 100% NBO. Further experimental studies should be performed to elucidate the mechanism and beneficial effects of 60% NBO, as it is more cost-effective to use, compared with 100% NBO.

Aging-related tau astrogliopathy (ARTAG): not only tau phosphorylation in astrocytes

Aging-related tau astrogliopathy (ARTAG) is defined by the presence of two types of tau-bearing astrocytes: thorn-shaped astrocytes (TSAs) and granular/fuzzy astrocytes in the brain of old-aged individuals. The present study is focused on TSAs in rare forms of ARTAG with no neuronal tau pathology or restricted to entorhinal and transentorhinal cortices, to avoid bias from associated tauopathies. TSAs show 4Rtau phosphorylation at several specific sites and abnormal tau conformation, but they lack ubiquitin and they are not immunostained with tau-C3 antibodies which recognize truncated tau at Asp421. Astrocytes in ARTAG have atrophic processes, reduced glial fibrillary acidic protein (GFAP) and increased superoxide dismutase 2 (SOD2) immunoreactivity. Gel electrophoresis and western blotting of sarkosyl-insoluble fractions reveal a pattern of phospho-tau in ARTAG characterized by two bands of 68 and 64 kDa, and several middle bands between 35 and 50 kDa which differ from what is seen in AD. Phosphoproteomics of dissected vulnerable regions identifies an increase of phosphorylation marks in a large number of proteins in ARTAG compared with controls. GFAP, aquaporin 4, several serine-threonine kinases, microtubule associated proteins and other neuronal proteins are among the differentially phosphorylated proteins in ARTAG thus suggesting a hyper-phosphorylation background that affects several molecules, including many kinases and proteins from several cell compartments and various cell types. Finally, present results show for the first time that tau seeding is produced in neurons of the hippocampal complex, astrocytes, oligodendroglia and along fibers of the corpus callosum, fimbria and fornix following inoculation into the hippocampus of wild type mice of sarkosyl-insoluble fractions enriched in hyper-phosphorylated tau from selected ARTAG cases. These findings show astrocytes as crucial players of tau seeding in tauopathies.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Noninvasive monitoring of brain edema after hypoxia in newborn piglets

BackgroundDevelopment of cerebral edema after brain injury carries a high risk for brain damage and death. The present study tests the ability of a noninvasive cerebral edema monitoring system that uses near-infrared spectroscopy (NIRS) with water as the chromophore of interest to detect brain edema following hypoxia.MethodsVentilated piglets were exposed to hypoxia for 1 h, and then returned to normal oxygen levels for 4 h. An NIRS sensor was placed on the animal's head at baseline, and changes in light attenuation were converted to changes in H₂O. Cerebral water content and aquaporin-4 protein (AQP4) expression were measured.ResultsThe system detected changes in NIRS-derived water signal as early as 2 h after hypoxia, and provided fivefold signal amplification, representing a 10% increase in brain water content and a sixfold increase in AQP4, 4 h after hypoxia. Changes in water signal correlated well with changes in cerebral water content (R=0.74) and AQP4 expression (R=0.97) in the piglet brain.ConclusionThe data show that NIRS can detect cerebral edema early in the injury process, thus providing an opportunity to initiate therapy at an earlier and more effective time-point after an insult than is available with current technology.

Overexpression of MicroRNA-145 Ameliorates Astrocyte Injury by Targeting Aquaporin 4 in Cerebral Ischemic Stroke. Biomed Res Int. 2017;2017:9530951. [PMID: 29057271 PMCID: PMC5615955 DOI: 10.1155/2017/9530951]

Cerebral ischemic stroke, which affects the global population, is a major disease with high incidence, mortality, and disability. Accumulating evidence has indicated that abnormal microRNA (miRNA) expression plays essential roles in the pathologies of ischemic stroke. Yet, the underlying regulatory mechanism of miRNAs in cerebral ischemic stroke remains unclear. We investigated the role of miR-145 in cerebral ischemic stroke and its potential mechanism in a model using primary cultured astrocytes. We detected the expression levels of miR-145 and its target gene AQP4 and assessed the role of miR-145 in cell death and apoptosis caused by oxygen-glucose deprivation (OGD). Bioinformatics analysis was used to explore the targets of miR-145. miR-145 expression levels were significantly decreased in primary astrocytes subjected to OGD. miR-145 overexpression promoted astrocyte health and inhibited OGD-induced apoptosis. AQP4 was a direct target of miR-145, and miR-145 suppressed AQP4 expression. Moreover, AQP4 enhanced astrocyte injury in ischemic stroke, and AQP4 knockdown diminished the miR-145-mediated protective effect on ischemic injury. Taken together, our results show that miR-145 plays an important role in protecting astrocytes from ischemic injury by downregulating AQP4 expression. These findings may highlight a novel therapeutic target in cerebral ischemic stroke.

Human and mouse cortical astrocytes differ in aquaporin-4 polarization toward microvessels

Aquaporin‐4 (AQP4), the predominant water channel in the brain, is expressed in astrocytes and ependymal cells. In rodents AQP4 is highly polarized to perivascular astrocytic endfeet and loss of AQP4 polarization is associated with disease. The present study was undertaken to compare the expression pattern of AQP4 in human and mouse cortical astrocytes. Cortical tissue specimens were sampled from 11 individuals undergoing neurosurgery wherein brain tissue was removed as part of the procedure, and compared with cortical tissue from 5 adult wild‐type mice processed similarly. The tissue samples were immersion‐fixed and prepared for AQP4 immunogold electron microscopy, allowing quantitative assessment of AQP4's subcellular distribution. In mouse we found that AQP4 water channels were prominently clustered around vessels, being 5 to 10‐fold more abundant in astrocytic endfoot membranes facing the capillary endothelium than in parenchymal astrocytic membranes. In contrast, AQP4 was markedly less polarized in human astrocytes, being only two to three‐fold enriched in astrocytic endfoot membranes adjacent to capillaries. The lower degree of AQP4 polarization in human subjects (1/3 of that in mice) was mainly due to higher AQP4 expression in parenchymal astrocytic membranes. We conclude that there are hitherto unrecognized species differences in AQP4 polarization toward microvessels in the cerebral cortex.

Aquaporins in Nervous System

Aquaporins (AQPs ) mediate water flux between the four distinct water compartments in the central nervous system (CNS). In the present chapter, we mainly focus on the expression and function of the 9 AQPs expressed in the CNS, which include five members of aquaporin subfamily: AQP1, AQP4, AQP5, AQP6, and AQP8; three members of aquaglyceroporin subfamily: AQP3, AQP7, and AQP9; and one member of superaquaporin subfamily: AQP11. In addition, AQP1, AQP2 and AQP4 expressed in the peripheral nervous system (PNS) are also reviewed. AQP4, the predominant water channel in the CNS, is involved both in the astrocyte swelling of cytotoxic edema and the resolution of vasogenic edema, and is of pivotal importance in the pathology of brain disorders such as neuromyelitis optica , brain tumors and Alzheimer's disease. Other AQPs are also involved in a variety of important physiological and pathological process in the brain. It has been suggested that AQPs could represent an important target in treatment of brain disorders like cerebral edema. Future investigations are necessary to elucidate the pathological significance of AQPs in the CNS.

Unified model of brain tissue microstructure dynamically binds diffusion and osmosis with extracellular space geometry

We present a universal model of brain tissue microstructure that dynamically links osmosis and diffusion with geometrical parameters of brain extracellular space (ECS). Our model robustly describes and predicts the nonlinear time dependency of tortuosity (λ=sqrt[D/D^{*}]) changes with very high precision in various media with uniform and nonuniform osmolarity distribution, as demonstrated by previously published experimental data (D = free diffusion coefficient, D^{*} = effective diffusion coefficient). To construct this model, we first developed a multiscale technique for computationally effective modeling of osmolarity in the brain tissue. Osmolarity differences across cell membranes lead to changes in the ECS dynamics. The evolution of the underlying dynamics is then captured by a level set method. Subsequently, using a homogenization technique, we derived a coarse-grained model with parameters that are explicitly related to the geometry of cells and their associated ECS. Our modeling results in very accurate analytical approximation of tortuosity based on time, space, osmolarity differences across cell membranes, and water permeability of cell membranes. Our model provides a unique platform for studying ECS dynamics not only in physiologic conditions such as sleep-wake cycles and aging but also in pathologic conditions such as stroke, seizure, and neoplasia, as well as in predictive pharmacokinetic modeling such as predicting medication biodistribution and efficacy and novel biomolecule development and testing.

Aquaporin-4 and spinal cord injury

Edema formation is a major problem following traumatic spinal cord injury (SCI) that acts to exacerbate secondary damage. Severity of edema correlates with reduced neurological outcome in human patients. To date, there are no effective treatments to directly resolve edema within the spinal cord. The aquaporin-4 (AQP4) water channel is found on membranes of astrocytic endfeet in direct contact with blood vessels, the glia limitans in contact with the cerebrospinal fluid and ependyma around the central canal. Being so locally expressed at the interface between fluid and tissue allow AQP4 channels to play an important role in the bidirectional regulation of water homeostasis under normal conditions and following trauma. With the need to better understand the pathophysiology underlying the devastating cellular events in SCI, animal models have become an integral part of exploration. Inevitably, several injury models have been developed (contusion, compression, transection) resulting in difficult interpretation between studies with conflicting results. This is true in the case of understanding the role of AQP4 in the progression and resolution of edema following SCI, whose role is still not completely understood and is highly dependent on the type of edema present (vasogenic vs cytotoxic). Here, we discuss regulation of AQP4 in varying injury models and the effects of potential therapeutic interventions on expression, edema formation and functional recovery. Better understanding of the precise role of AQP4 following a wide range of injuries will help to understand optimal treatment timing following human SCI for prime therapeutic benefit and enhanced neurological outcome.

Relationship of Grafted FGF-2-Overexpressing Neural Stem/Progenitor Cells With the Vasculature in the Cerebral Cortex

Neural progenitor cells (NPCs) overexpressing fibroblast growth factor 2 (FGF-2) have the distinct tendency to associate with the vasculature and establish multiple proliferative clusters in the perivascular environment after transplantation into the cerebral cortex. Strikingly, the vascular clusters of progenitor cells give rise to immature neurons after ischemic injury, raising prospects for the formation of ectopic neurogenic niches for repair. We investigated the spatial relationship of perivascular clusters with the host vascular structures. FGF-2-GFP-transduced NPCs were transplanted into the intact somatosensory rat cortex. Confocal microscopic analysis revealed that grafted cells preferentially contacted venules at sites with aquaporin-4-positive astrocytic endfeet and avoided contacts with desmin-positive pericytes. Electron microscopic analysis confirmed that grafted cells preferentially made contact with astroglial endfeet, and only a minority of them reached the endothelial basal lamina. These results provide new insights into the fine structural and anatomical relationship between grafted FGF-2-transduced NPCs and the host vasculature.