Immunological function of aquaporin-4 in stab-wounded mouse brain in concert with a pro-inflammatory cytokine inducer, osteopontin

The deletion of AQP4 and TRPV4 affects astrocyte swelling/volume recovery in response to ischemia-mimicking pathologies

Introduction

Astrocytic Transient receptor potential vanilloid 4 (TRPV4) channels, together with Aquaporin 4 (AQP4), are suspected to be the key players in cellular volume regulation, and therefore may affect the development and severity of cerebral edema during ischemia. In this study, we examined astrocytic swelling/volume recovery in mice with TRPV4 and/or AQP4 deletion in response to in vitro ischemic conditions, to determine how the deletion of these channels can affect the development of cerebral edema.

Methods

We used three models of ischemia-related pathological conditions: hypoosmotic stress, hyperkalemia, and oxygenglucose deprivation (OGD), and observed their effect on astrocyte volume changes in acute brain slices of Aqp4-/-, Trpv4-/- and double knockouts. In addition, we employed single-cell RT-qPCR to assess the effect of TRPV4 and AQP4 deletion on the expression of other ion channels and transporters involved in the homeostatic functioning of astrocytes.

Results

Quantification of astrocyte volume changes during OGD revealed that the deletion of AQP4 reduces astrocyte swelling, while simultaneous deletion of both AQP4 and TRPV4 leads to a disruption of astrocyte volume recovery during the subsequent washout. Of note, astrocyte exposure to hypoosmotic stress or hyperkalemia revealed no differences in astrocyte swelling in the absence of AQP4, TRPV4, or both channels. Moreover, under ischemia-mimicking conditions, we identified two distinct subpopulations of astrocytes with low and high volumetric responses (LRA and HRA), and their analyses revealed that mainly HRA are affected by the deletion of AQP4, TRPV4, or both channels. Furthermore, gene expression analysis revealed reduced expression of the ion transporters KCC1 and ClC2 as well as the receptors GABAB and NMDA in Trpv4-/- mice. The deletion of AQP4 instead caused reduced expression of the serine/cysteine peptidase inhibitor Serpina3n.

Discussion

Thus, we showed that in AQP4 or TRPV4 knockouts, not only the specific function of these channels is affected, but also the expression of other proteins, which may modulate the ischemic cascade and thus influence the final impact of ischemia.

Aquaporin-4 and transient receptor potential vanilloid 4 balance in early postnatal neurodevelopment

In the adult brain, the water channel aquaporin-4 (AQP4) is expressed in astrocyte endfoot, in supramolecular assemblies, called "Orthogonal Arrays of Particles" (OAPs) together with the transient receptor potential vanilloid 4 (TRPV4), finely regulating the cell volume. The present study aimed at investigating the contribution of AQP4 and TRPV4 to CNS early postnatal development using WT and AQP4 KO brain and retina and neuronal stem cells (NSCs), as an in vitro model of astrocyte differentiation. Western blot analysis showed that, differently from AQP4 and the glial cell markers, TRPV4 was downregulated during CNS development and NSC differentiation. Blue native/SDS-PAGE revealed that AQP4 progressively organized into OAPs throughout the entire differentiation process. Fluorescence quenching assay indicated that the speed of cell volume changes was time-related to NSC differentiation and functional to their migratory ability. Calcium imaging showed that the amplitude of TRPV4 Ca2+ transient is lower, and the dynamics are changed during differentiation and suppressed in AQP4 KO NSCs. Overall, these findings suggest that early postnatal neurodevelopment is subjected to temporally modulated water and Ca2+ dynamics likely to be those sustaining the biochemical and physiological mechanisms responsible for astrocyte differentiation during brain and retinal development.

The Role of Aquaporins in Epileptogenesis-A Systematic Review

Aquaporins (AQPs) are a family of membrane proteins involved in the transport of water and ions across cell membranes. AQPs have been shown to be implicated in various physiological and pathological processes in the brain, including water homeostasis, cell migration, and inflammation, among others. Epileptogenesis is a complex and multifactorial process that involves alterations in the structure and function of neuronal networks. Recent evidence suggests that AQPs may also play a role in the pathogenesis of epilepsy. In animal models of epilepsy, AQPs have been shown to be upregulated in regions of the brain that are involved in seizure generation, suggesting that they may contribute to the hyperexcitability of neuronal networks. Moreover, genetic studies have identified mutations in AQP genes associated with an increased risk of developing epilepsy. Our review aims to investigate the role of AQPs in epilepsy and seizure onset from a pathophysiological point of view, pointing out the potential molecular mechanism and their clinical implications.

Stimulated Raman scattering microscopy reveals a unique and steady nature of brain water dynamics

The biological activities of substances in the brain are shaped by their spatiotemporal dynamics in brain tissues, all of which are regulated by water dynamics. In contrast to solute dynamics, water dynamics have been poorly characterized, owing to the lack of appropriate analytical tools. To overcome this limitation, we apply stimulated Raman scattering multimodal multiphoton microscopy to live brain tissues. The microscopy system allows for the visualization of deuterated water, fluorescence-labeled solutes, and cellular structures at high spatiotemporal resolution, revealing that water moves faster than fluorescent molecules in brain tissues. Detailed analyses demonstrate that water, unlike solutes, diffuses homogeneously in brain tissues without differences between the intra- and the extracellular routes. Furthermore, we find that the water dynamics are steady during development and ischemia, when diffusions of solutes are severely affected. Thus, our approach reveals routes and uniquely robust properties of water diffusion in brain tissues.

Toward discovering a novel family of peptides targeting neuroinflammatory states of brain microglia and astrocytes

Microglia are immune-derived cells critical to the development and healthy function of the brain and spinal cord, yet are implicated in the active pathology of many neuropsychiatric disorders. A range of functional phenotypes associated with the healthy brain or disease states has been suggested from in vivo work and were modeled in vitro as surveying, reactive, and primed sub-types of primary rat microglia and mixed microglia/astrocytes. It was hypothesized that the biomolecular profile of these cells undergoes a phenotypical change as well, and these functional phenotypes were explored for potential novel peptide binders using a custom 7 amino acid-presenting M13 phage library (SX7) to identify unique peptides that bind differentially to these respective cell types. Surveying glia were untreated, reactive were induced with a lipopolysaccharide treatment, recovery was modeled with a potent anti-inflammatory treatment dexamethasone, and priming was determined by subsequently challenging the cells with interferon gamma. Microglial function was profiled by determining the secretion of cytokines and nitric oxide, and expression of inducible nitric oxide synthase. After incubation with the SX7 phage library, populations of SX7-positive microglia and/or astrocytes were collected using fluorescence-activated cell sorting, SX7 phage was amplified in Escherichia coli culture, and phage DNA was sequenced via next-generation sequencing. Binding validation was done with synthesized peptides via in-cell westerns. Fifty-eight unique peptides were discovered, and their potential functions were assessed using a basic local alignment search tool. Peptides potentially originated from proteins ranging in function from a variety of supportive glial roles, including synapse support and pruning, to inflammatory incitement including cytokine and interleukin activation, and potential regulation in neurodegenerative and neuropsychiatric disorders.

Prolonged Light Exposure Induces Circadian Impairment in Aquaporin-4-Knockout Mice

Astrocytes are densely present in the suprachiasmatic nucleus (SCN), which is the master circadian oscillator in mammals, and are presumed to play a key role in circadian oscillation. However, specific astrocytic molecules that regulate the circadian clock are not yet well understood. In our study, we found that the water channel aquaporin-4 (AQP4) was abundantly expressed in SCN astrocytes, and we further examined its circadian role using AQP4-knockout mice. There was no prominent difference in circadian behavioral rhythms between Aqp4^-/- and Aqp4^+/+ mice subjected to light-dark cycles and constant dark conditions. However, exposure to constant light induced a greater decrease in the Aqp4^-/- mice rhythmicity. Although the damped rhythm in long-term constant light recovered after transfer to constant dark conditions in both genotypes, the period until the reappearance of original rhythmicity was severely prolonged in Aqp4^-/- mice. In conclusion, AQP4 absence exacerbates the prolonged light-induced impairment of circadian oscillations and delays their recovery to normal rhythmicity.

2-carba-cyclic phosphatidic acid modulates astrocyte-to-microglia communication and influences microglial polarization towards an anti-inflammatory phenotype

2-carba-cyclic phosphatidic acid (2ccPA) suppresses microglial and astrocyte inflammation for neuronal survival following traumatic brain injury. However, it remains unknown how 2ccPA regulates microglial activation. In this study, to elucidate the 2ccPA behavior in glial communication, we collected the astrocyte conditioned media (ACM) from primary astrocyte cultures that were treated by lipopolysaccharide (LPS) and 2ccPA and analyzed the alteration of microglial inflammation caused by the ACM treatment. The addition of the ACM derived from LPS- and 2ccPA-double treated astrocytes to microglia decreased the CD86⁺ pro-inflammatory M1 microglia, which were upregulated with the ACM collected from astrocytes treated by LPS without 2ccPA, while the direct addition of LPS and 2ccPA to microglia failed to decrease the CD86⁺ microglia to the basal level. We confirmed that the ACM from LPS- and 2ccPA-treated astrocytes increased the ratio of CD206⁺ anti-inflammatory M2 microglia to total microglia, whereas direct treatment of microglia with LPS and 2ccPA had no effect on the CD206⁺ microglia ratio, demonstrating the importance of astrocyte intervention in microglial polarization. In addition, we examined whether astrocytes modulate the 2ccPA-regulated proinflammatory cytokine production derived from microglia. The addition of the ACM from LPS- and 2ccPA-treated astrocytes to microglia remarkably canceled the LPS-induced upregulation of IL-1β, IL-6, and TNF-α secreted from microglia, while the direct addition of LPS and 2ccPA to microglia showed no affect. Therefore, our results indicate that astrocytes mediate the 2ccPA function to shift microglia towards the M2 phenotype by interfering with the polarization of M1 microglia and to suppress cytokine production.

Astrocytic Neuroimmunological Roles Interacting with Microglial Cells in Neurodegenerative Diseases

Both astrocytic and microglial functions have been extensively investigated in healthy subjects and neurodegenerative diseases. For astrocytes, not only various sub-types were identified but phagocytic activity was also clarified recently and is making dramatic progress. In this review paper, we mostly focus on the functional role of astrocytes in the extracellular matrix and on interactions between reactive astrocytes and reactive microglia in normal states and in neurodegenerative diseases, because the authors feel it is necessary to elucidate the mechanisms among activated glial cells in the pathology of neurological diseases in order to pave the way for drug discovery. Finally, we will review cyclic phosphatidic acid (cPA), a naturally occurring phospholipid mediator that induces a variety of biological activities in the brain both in vivo and in vitro. We propose that cPA may serve as a novel therapeutic molecule for the treatment of brain injury and neuroinflammation.

Distribution of intraperitoneally administered deuterium-labeled water in aquaporin-4-knockout mouse brain after middle cerebral artery occlusion

Introduction

As the movement of water in the brain is known to be involved in neural activity and various brain pathologies, the ability to assess water dynamics in the brain will be important for the understanding of brain function and the diagnosis and treatment of brain diseases. Aquaporin-4 (AQP4) is a membrane channel protein that is highly expressed in brain astrocytes and is important for the movement of water molecules in the brain.

Methods

In this study, we investigated the contribution of AQP4 to brain water dynamics by administering deuterium-labeled water (D₂O) intraperitoneally to wild-type and AQP4 knockout (AQP4-ko) mice that had undergone surgical occlusion of the middle cerebral artery (MCA). Water dynamics in the infarct region and on either side of the anterior cerebral artery (ACA) was monitored with proton-density-weighted imaging (PDWI) performed on a 7T animal MRI.

Results

D₂O caused a negative signal change quickly after administration. The AQP4-ko mice showed a delay of the time-to-minimum in both the contralateral and ipsilateral ACA regions compared to wild-type mice. Also, only the AQP4- ko mice showed a delay of the time-to-minimum in the ipsilateral ACA region compared to the contralateral side. In only the wild-type mice, the signal minimum in the ipsilateral ACA region was higher than that in the contralateral ACA region. In the infarct region, the signal attenuation was slower for the AQP4-ko mice in comparison to the wild-type mice.

Discussion

These results suggest that AQP4 loss affects water dynamics in the ACA region not only in the infarct region. Dynamic PDWI after D₂O administration may be a useful tool for showing the effects of AQP4 in vivo.

Rho-Kinase inhibition decreases focal cerebral ischemia-induced glial activation in rats

Background

Rho-kinase inhibition in a rat middle cerebral artery occlusion (MCAO) model is reported to improve neurological functions and decrease infarction size.

Objective

The objective of this study is to investigate the underlying mechanisms of such improvement by evaluating the effects of Rho-kinase inhibition on astrocytes and microglial accumulation and activation in this condition.

Methods

Adult male Sprague-Dawley (SD) rats were used to generate the MCAO model, which received an I.P injection of a chemical Rho-kinase inhibitor (Fasudil- 5 mg/kg/day) or vehicle (PBS) for 2 and 4 days.

Results

Fasudil treatment significantly decreased the stroke volumes and water content in the lesion areas, as revealed by MRI. Immunostaining and Western blotting results demonstrated that Fasudil significantly decreased the levels of Aquaporin-4, a water channel protein. The number of GFAP⁺ astrocytes and Iba-1⁺ macrophage/microglia was decreased in the lesion areas. Proinflammatory transcription factor NF-κB protein levels were decreased in the Fasudil group 2 days after MCAO. Also, proinflammatory mediators including TNF-α, IL-1β, and iNOS levels were decreased. In vitro migration study using a human microglial cell line (HMO6) confirmed the inhibitory effects of Fasudil on the process. Fasudil also decreased combined IL-1β and IFNγ-induced NF-κB nuclear translocation in HMO6. Moreover, Fasudil transiently decreased combined IL-1β and IFNγ-induced iNOS, TNFα, and IL-1β mRNA levels in HMO6.

Conclusion

Our study demonstrates the inhibitory effects of Rho-kinase on NF-κB-mediated glial activation and cerebral edema, which might be a promising therapeutic target in acute cerebral ischemia conditions.

Exploring the role of astrocytic dysfunction and AQP4 in depression

Aquaporin-4 (AQP4) is the water regulating channel found in the terminal processes of astrocytes in the brain and is implicated in regulating the astrocyte functions, whereas in neuropathologies, AQP4 performs an important role in astrocytosis and release of proinflammatory cytokines. However, several findings have revealed the modulation of the AQP4 water channel in the etiopathogenesis of various neuropsychiatric diseases. In the current article, we have summarized the recent studies and highlighted the implication of astrocytic dysfunction and AQP4 in the etiopathogenesis of depressive disorder. Most of the studies have measured the AQP4 gene or protein expression in the brain regions, particularly the locus coeruleus, choroid plexus, prefrontal cortex, and hippocampus, and found that in these brain regions, AQP4 gene expression decreased on exposure to chronic mild stress. Few studies also measured the peripheral AQP4 mRNA expression in the blood and AQP4 autoantibodies in the blood serum and revealed no change in the depressed patients in comparison with normal individuals.

Aquaporin-4 in Neuromyelitis Optica Spectrum Disorders: A Target of Autoimmunity in the Central Nervous System

Since the discovery of a specific autoantibody in patients with neuromyelitis optica spectrum disorder (NMOSD) in 2004, the water channel aquaporin-4 (AQP4) has attracted attention as a target of autoimmune diseases of the central nervous system. In NMOSD, the autoantibody (NMO-IgG) binds to the extracellular loops of AQP4 as expressed in perivascular astrocytic end-feet and disrupts astrocytes in a complement-dependent manner. NMO-IgG is an excellent marker for distinguishing the disease from other inflammatory demyelinating diseases, such as multiple sclerosis. The unique higher-order structure of AQP4—called orthogonal arrays of particles (OAPs)—as well as its subcellular localization may play a crucial role in the pathogenesis of the disease. Recent studies have also demonstrated complement-independent cytotoxic effects of NMO-IgG. Antibody-induced endocytosis of AQP4 has been suggested to be involved in this mechanism. This review focuses on the binding properties of antibodies that recognize the extracellular region of AQP4 and the characteristics of AQP4 that are implicated in the pathogenesis of NMOSD.

Glymphatic system clears extracellular tau and protects from tau aggregation and neurodegeneration

Accumulation of tau has been implicated in various neurodegenerative diseases termed tauopathies. Tau is a microtubule-associated protein but is also actively released into the extracellular fluids including brain interstitial fluid and cerebrospinal fluid (CSF). However, it remains elusive whether clearance of extracellular tau impacts tau-associated neurodegeneration. Here, we show that aquaporin-4 (AQP4), a major driver of the glymphatic clearance system, facilitates the elimination of extracellular tau from the brain to CSF and subsequently to deep cervical lymph nodes. Strikingly, deletion of AQP4 not only elevated tau in CSF but also markedly exacerbated phosphorylated tau deposition and the associated neurodegeneration in the brains of transgenic mice expressing P301S mutant tau. The current study identified the clearance pathway of extracellular tau in the central nervous system, suggesting that glymphatic clearance of extracellular tau is a novel regulatory mechanism whose impairment contributes to tau aggregation and neurodegeneration.

Integrin Signaling in the Central Nervous System in Animals and Human Brain Diseases

The integrin family is involved in various biological functions, including cell proliferation, differentiation and migration, and also in the pathogenesis of disease. Integrins are multifunctional receptors that exist as heterodimers composed of α and β subunits and bind to various ligands, including extracellular matrix (ECM) proteins; they are found in many animals, not only vertebrates (e.g., mouse, rat, and teleost fish), but also invertebrates (e.g., planarian flatworm, fruit fly, nematodes, and cephalopods), which are used for research on genetics and social behaviors or as models for human diseases. In the present paper, we describe the results of a phylogenetic tree analysis of the integrin family among these species. We summarize integrin signaling in teleost fish, which serves as an excellent model for the study of regenerative systems and possesses the ability for replacing missing tissues, especially in the central nervous system, which has not been demonstrated in mammals. In addition, functions of astrocytes and reactive astrocytes, which contain neuroprotective subpopulations that act in concert with the ECM proteins tenascin C and osteopontin via integrin are also reviewed. Drug development research using integrin as a therapeutic target could result in breakthroughs for the treatment of neurodegenerative diseases and brain injury in mammals.

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.

The α-dystrobrevins play a key role in maintaining the structure and function of the extracellular matrix-significance for protein elimination failure arteriopathies

The extracellular matrix (ECM) of the cerebral vasculature provides a pathway for the flow of interstitial fluid (ISF) and solutes out of the brain by intramural periarterial drainage (IPAD). Failure of IPAD leads to protein elimination failure arteriopathies such as cerebral amyloid angiopathy (CAA). The ECM consists of a complex network of glycoproteins and proteoglycans that form distinct basement membranes (BM) around different vascular cell types. Astrocyte endfeet that are localised against the walls of blood vessels are tethered to these BMs by dystrophin associated protein complex (DPC). Alpha-dystrobrevin (α-DB) is a key dystrophin associated protein within perivascular astrocyte endfeet; its deficiency leads to a reduction in other dystrophin associated proteins, loss of AQP4 and altered ECM. In human dementia cohorts there is a positive correlation between dystrobrevin gene expression and CAA. In the present study, we test the hypotheses that (a) the positive correlation between dystrobrevin gene expression and CAA is associated with elevated expression of α-DB at glial-vascular endfeet and (b) a deficiency in α-DB results in changes to the ECM and failure of IPAD. We used human post-mortem brain tissue with different severities of CAA and transgenic α-DB deficient mice. In human post-mortem tissue we observed a significant increase in vascular α-DB with CAA (CAA vrs. Old p < 0.005, CAA vrs. Young p < 0.005). In the mouse model of α-DB deficiency, there was early modifications to vascular ECM (collagen IV and BM thickening) that translated into reduced IPAD efficiency. Our findings highlight the important role of α-DB in maintaining structure and function of ECM, particularly as a pathway for the flow of ISF and solutes out of the brain by IPAD.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Urine proteome changes in a chronic unpredictable mild stress (CUMS) mouse model of major depressive disorder

Major depressive disorder (MDD) is a prevalent complex psychiatric disorder, and there are no effective biomarkers for clinical diagnosis. Urine is not subjected to homeostatic control, allowing it to reflect the sensitive changes that occur in various diseases. In this study, we examined the urine proteome changes in a chronic unpredictable mild stress mouse model of MDD. Male C57BL/6 mice were subjected to chronic unpredictable mild stress for 5 weeks. The tail suspension test and sucrose consumption test were then applied to evaluate depression-like behaviors. The urine proteomes on day 0 and day 36 in the CUMS group were profiled by liquid chromatography coupled with tandem mass spectrometry (LCMS/MS). A total of 36 differential proteins were identified, 19 of which have been associated with the pathogenic mechanisms of MDD. There was an average of two differential proteins that were identified through 1,048,574 random combination statistical analyses, indicating that at least 95 % of the differential proteins were reliable. The differential proteins were mainly associated with blood coagulation, inflammatory responses and central nervous system development. Our preliminary results indicated that the urine proteome can reflect changes associated with MDD in the CUMS model, which provides potential clues for the diagnosis of MDD.

Glymphatic system, AQP4, and their implications in Alzheimer's disease

Lacking conventional lymphatic system, the central nervous system requires alternative clearance systems, such as the glymphatic system, which promotes clearance of interstitial solutes. Aquaporin-4 water channels (AQP4) are an integral part of this system and related to neuropathologies, such as Alzheimer's disease (AD). The clearance of Alzheimer's associated proteins amyloid β and tau is diminished by glymphatic system impairment, due to lack of AQP4. Even though AQP4 mislocalisation (which affects its activity) is a phenotype of AD, it remains a controversial topic, as it is still unclear if it is a phenotype-promoting factor or a consequence of this pathology. This review provides important and updated knowledge about glymphatic system, AQP4 itself, and their link with Alzheimer's disease. Finally, AQP4 as a therapeutic target is proposed to ameliorate Alzheimer's Disease and other neuropathologies AQP4-related.

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

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

Behavioral and electrophysiological evidence for a neuroprotective role of aquaporin-4 in the 5xFAD transgenic mice model

Aquaporin-4 (AQP4) has been suggested to be involved in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD), which may be due to the modulation of neuroinflammation or the impairment of interstitial fluid bulk flow system in the central nervous system. Here, we show an age-dependent impairment of several behavioral outcomes in 5xFAD AQP4 null mice. Twenty-four-hour video recordings and computational analyses of their movement revealed that the nighttime motion of AQP4-deficient 5xFAD mice was progressively reduced between 20 and 36 weeks of age, with a sharp deterioration occurring between 30 and 32 weeks. This reduction in nighttime motion was accompanied by motor dysfunction and epileptiform neuronal activities, demonstrated by increased abnormal spikes by electroencephalography. In addition, all AQP4-deficient 5xFAD mice exhibited convulsions at least once during the period of the analysis. Interestingly, despite such obvious phenotypes, parenchymal amyloid β (Aβ) deposition, reactive astrocytosis, and activated microgliosis surrounding amyloid plaques were unchanged in the AQP4-deficient 5xFAD mice relative to 5xFAD mice. Taken together, our data indicate that AQP4 deficiency greatly accelerates an age-dependent deterioration of neuronal function in 5xFAD mice associated with epileptiform neuronal activity without significantly altering Aβ deposition or neuroinflammation in this mouse model. We therefore propose that there exists another pathophysiological phase in AD which follows amyloid plaque deposition and neuroinflammation and is sensitive to AQP4 deficiency.

Glial Factors Regulating White Matter Development and Pathologies of the Cerebellum

The cerebellum is a brain region that undergoes extremely dynamic growth during perinatal and postnatal development which is regulated by the proper interaction between glial cells and neurons with a complex concert of growth factors, chemokines, cytokines, neurotransmitters and transcriptions factors. The relevance of cerebellar functions for not only motor performance but also for cognition, emotion, memory and attention is increasingly being recognized and acknowledged. Since perturbed circuitry of cerebro-cerebellar trajectories can play a role in many central nervous system pathologies and thereby contribute to neurological symptoms in distinct neurodevelopmental and neurodegenerative diseases, is it the aim with this mini-review to highlight the pathways of glia–glia interplay being involved. The designs of future treatment strategies may hence be targeted to molecular pathways also playing a role in development and disease of the cerebellum.

Aquaporin 4 Suppresses Neural Hyperactivity and Synaptic Fatigue and Fine-Tunes Neurotransmission to Regulate Visual Function in the Mouse Retina

The bidirectional water channel aquaporin 4 (AQP4) is abundantly expressed in the neural tissue. The advantages and disadvantages of AQP4 neural tissue deficiency under pathological conditions, such as inflammation, and relationship with neural diseases, such as Alzheimer’s disease, have been previously reported. However, the physiological functions of AQP4 are not fully understood. Here, we evaluated the role of AQP4 in the mouse retina using Aqp4 knockout (KO) mice. Aqp4 was expressed in Müller glial cells surrounding the synaptic area between photoreceptors and bipolar cells. Both scotopic and photopic electroretinograms showed hyperactive visual responses in KO mice, gradually progressing with age. Moreover, the amplitude reduction after frequent stimuli and synaptic fatigue was more severe in KO mice. Glutamine synthetase, glutamate aspartate transporter, synaptophysin, and the inward potassium channel Kir2.1, but not Kir4.1, were downregulated in KO retinas. KIR2.1 colocalized with AQP4 in Müller glial cells at the synaptic area, and its expression was affected by Aqp4 levels in primary Müller glial cell cultures. Intraocular injection of potassium in wild-type mice led to visual function hyperactivity, as observed in Aqp4 KO mice. Mitochondria molecules, such as Pgc1α and CoxIV, were downregulated, while apoptotic markers were upregulated in KO retinas. AQP4 may fine-tune synaptic activity, most likely by regulating potassium metabolism, at least in part, via collaborating with KIR2.1, and possibly indirectly regulating glutamate kinetics, to inhibit neural hyperactivity and synaptic fatigue which finally affect mitochondria and cause neurodegeneration.

Adrenergic receptor antagonism induces neuroprotection and facilitates recovery from acute ischemic stroke

Spontaneous waves of cortical spreading depolarization (CSD) are induced in the setting of acute focal ischemia. CSD is linked to a sharp increase of extracellular K⁺ that induces a long-lasting suppression of neural activity. Furthermore, CSD induces secondary irreversible damage in the ischemic brain, suggesting that K⁺ homeostasis might constitute a therapeutic strategy in ischemic stroke. Here we report that adrenergic receptor (AdR) antagonism accelerates normalization of extracellular K⁺, resulting in faster recovery of neural activity after photothrombotic stroke. Remarkably, systemic adrenergic blockade before or after stroke facilitated functional motor recovery and reduced infarct volume, paralleling the preservation of the water channel aquaporin-4 in astrocytes. Our observations suggest that AdR blockers promote cerebrospinal fluid exchange and rapid extracellular K⁺ clearance, representing a potent potential intervention for acute stroke.

Vitronectin deficiency attenuates hepatic fibrosis in a non-alcoholic steatohepatitis-induced mouse model

Vitronectin (VN), an extracellular matrix protein, is a promising immune biomarker of non-alcoholic steatohepatitis (NASH); however, its precise function remains unclear. This study investigated how VN deficiency contributes to the development of NASH. Towards this aim, wild-type (WT) and VN-/- mice were fed with a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD) for 6 and 10 weeks to induce NASH, and the livers were isolated. In WT mice fed with CDAHFD for 6 and 10 weeks, the expression of Vn mRNA and protein was up-regulated compared with that in mice fed with the MF control diet, indicating that VN is regulated in NASH condition. VN-/- mice showed decreased picrosirius red staining in the liver area and Col1a2 mRNA expression levels, compared with WT mice, indicating that the severity of hepatic fibrosis is attenuated in the CDAHFD-fed VN-/- mice. In addition, VN deficiency did not affect the area of lipid droplets in haematoxylin-eosin staining and the mRNA expression levels of fatty acid synthases, Srebp, Acc and Fas in the CDAHFD-fed mice. Moreover, VN deficiency decreased the inflammation score and the mRNA expression levels of Cd11b and F4/80, macrophage markers, as well as Tnf-α and Il-1β, inflammatory cytokines in the CDAHFD-fed mice. Furthermore, VN deficiency decreased the protein and mRNA expression levels of α-smooth muscle actin in the CDAHFD-fed mice, suggesting that VN deficiency inhibits the activation of hepatic stellate cells (HSCs). Our findings indicate that VN contributes to the development of fibrosis in the NASH model mice via modulation of the inflammatory reaction and activation of HSCs.

The role of aquaporin-4 and transient receptor potential vaniloid isoform 4 channels in the development of cytotoxic edema and associated extracellular diffusion parameter changes

The proper function of the nervous system is dependent on the balance of ions and water between the intracellular and extracellular space (ECS). It has been suggested that the interaction of aquaporin-4 (AQP4) and the transient receptor potential vaniloid isoform 4 (TRPV4) channels play a role in water balance and cell volume regulation, and indirectly, of the ECS volume. Using the real-time iontophoretic method, we studied the changes of the ECS diffusion parameters: ECS volume fraction α (α = ECS volume fraction/total tissue volume) and tortuosity λ (λ²  = free/apparent diffusion coefficient) in mice with a genetic deficiency of AQP4 or TRPV4 channels, and in control animals. The used models of cytotoxic edema included: mild and severe hypotonic stress or oxygen-glucose deprivation (OGD) in situ and terminal ischemia/anoxia in vivo. This study shows that an AQP4 or TRPV4 deficit slows down the ECS volume shrinkage during severe ischemia in vivo. We further demonstrate that a TRPV4 deficit slows down the velocity and attenuates an extent of the ECS volume decrease during OGD treatment in situ. However, in any of the cytotoxic edema models in situ (OGD, mild or severe hypotonic stress), we did not detect any alterations in the cell swelling or volume regulation caused by AQP4 deficiency. Overall, our results indicate that the AQP4 and TRPV4 channels may play a crucial role in severe pathological states associated with their overexpression and enhanced cell swelling. However, detailed interplay between AQP4 and TRPV4 channels requires further studies and additional research.

Aquaporin-4-dependent glymphatic solute transport in the rodent brain

The glymphatic system is a brain-wide clearance pathway; its impairment contributes to the accumulation of amyloid-β. Influx of cerebrospinal fluid (CSF) depends upon the expression and perivascular localization of the astroglial water channel aquaporin-4 (AQP4). Prompted by a recent failure to find an effect of Aqp4 knock-out (KO) on CSF and interstitial fluid (ISF) tracer transport, five groups re-examined the importance of AQP4 in glymphatic transport. We concur that CSF influx is higher in wild-type mice than in four different Aqp4 KO lines and in one line that lacks perivascular AQP4 (Snta1 KO). Meta-analysis of all studies demonstrated a significant decrease in tracer transport in KO mice and rats compared to controls. Meta-regression indicated that anesthesia, age, and tracer delivery explain the opposing results. We also report that intrastriatal injections suppress glymphatic function. This validates the role of AQP4 and shows that glymphatic studies must avoid the use of invasive procedures.

Osteopontin is indispensable for activation of astrocytes in injured mouse brain and primary culture

OBJECTIVES

Osteopontin (OPN) is an inflammatory cytokine inducer involved in cell proliferation and migration in inflammatory diseases or tumors. To investigate the function of OPN in astrocyte activation during brain injury, we compared OPN-deficient (OPN/KO) with wild-type (WT) mouse brains after stab wound injury and primary culture of astrocytes.

METHODS

Primary cultures of astrocytes were prepared from either WT or OPN/KO postnatal mouse brains. Activation efficiency of astrocytes in primary culture was accessed using Western blotting by examining the protein levels of glial fibrillary acidic protein (GFAP) and tenascin-C (TN-C), which are markers for reactive astrocytes, following lipopolysaccharide (LPS) stimulation. Furthermore, the stab wound injury on the cerebral cortex as a brain traumatic injury model was used, and activation of astrocytes and microglial cells was investigated using immunofluorescent analysis on fixed brain sections.

RESULTS

Primary cultures of astrocytes prepared from WT or OPN/KO postnatal mouse brains showed that only 25% of normal shaped astrocytes in a flask were produced in OPN/KO mice. The expression levels of both GFAP and TN-C were downregulated in the primary culture of astrocytes from OPN/KO mice compared with that from WT mice. By the immunofluorescent analysis on the injured brain sections, glial activation was attenuated in OPN/KO mice compared with WT mice.

DISCUSSION

Our data suggest that OPN is essential for proper astrocytic generation in vitro culture prepared from mouse cerebral cortex. OPN is indispensable for astrocyte activation in the mouse brain injury model and in LPS stimulated primary culture.

ABBREVIATIONS

AQP4: aquaporin 4; BBB: blood brain barrier; BrdU: bromo-deoxy uridine; CNS: central nervous system; GFAP: glial fibllirary acidic protein; IgG: immunoglobulin G; LPS: lipopolysaccharide; OPN: osteopontin; OPN/KO: osteopontin-deficient; TN-C: tenascin-C.

AQP4‑knockout alleviates the lipopolysaccharide‑induced inflammatory response in astrocytes via SPHK1/MAPK/AKT signaling

To date, aquaporin‑4 (AQP4) has been considered as a critical contributor to neuroinflammation, but little is known about the underlying mechanism. Previous studies have shown that a critical enzyme involved in the sphingomyelin cycle, sphingosine kinase 1 (SPHK1), is implicated in inflammatory processes and contributes to chronic neuroinflammation. The present study investigated the role of AQP4 in proinflammatory cytokine release from astrocytes, with an emphasis on the SPHK1/mitogen‑activated protein kinase (MAPK)/protein kinase B (AKT) pathway. Using primary cultures isolated from AQP4+/+ and AQP4‑/‑ embryos, the production of tumor necrosis factor‑α (TNF‑α)/interleukin‑6 (IL‑6) from astrocytes challenged by lipopolysaccharide (LPS) was compared. The results showed increased secretion of TNF‑α/IL‑6 in the two groups following LPS treatment, but a significantly lower level was observed in the AQP4‑/‑ group compared with that in the AQP4+/+ group. Although upregulation of SPHK1 was detected in the two genotypes, only a mild increase in SPHK1 was found in the AQP4‑/‑ genotype. The phosphorylation of MAPK/AKT was also confirmed to be attenuated in the AQP4‑/‑ group, suggesting decreased MAPK/AKT signaling over time in AQP4‑/‑ astrocytes. Overall, the study findings demonstrated that AQP4 deficiency alleviates proinflammatory cytokine release from astrocytes, in association with the SPHK1/MAPK/AKT pathway. This data improves our understanding of AQP4 in neuroinflammatory events, highlighting a novel profile of SPHK1 as a potential target for the treatment of CNS inflammation.

2-carba cyclic phosphatidic acid suppresses inflammation via regulation of microglial polarisation in the stab-wounded mouse cerebral cortex

Traumatic brain injury (TBI) is caused by physical damage to the brain and it induces blood-brain barrier (BBB) breakdown and inflammation. To diminish the sequelae of TBI, it is important to decrease haemorrhage and alleviate inflammation. In this study, we aimed to determine the effects of 2-carba-cyclic phosphatidic acid (2ccPA) on the repair mechanisms after a stab wound injury as a murine TBI model. The administration of 2ccPA suppressed serum immunoglobulin extravasation after the injury. To elucidate the effects of 2ccPA on inflammation resulting from TBI, we analysed the mRNA expression of inflammatory cytokines. We found that 2ccPA prevents a TBI-induced increase in the mRNA expression of Il-1β, Il-6, Tnf-α and Tgf-β1. In addition, 2ccPA reduces the elevation of Iba1 levels. These data suggest that 2ccPA attenuates the inflammation after a stab wound injury via the modulation of pro-inflammatory cytokines release from microglial cells. Therefore, we focused on the function of 2ccPA in microglial polarisation towards M1 or M2 phenotypes. The administration of 2ccPA decreased the number of M1 and increased the number of M2 type microglial cells, indicating that 2ccPA modulates the microglial polarisation and shifts them towards M2 phenotype. These data suggest that 2ccPA treatment suppresses the extent of BBB breakdown and inflammation after TBI.

Correlation between astrocyte activity and recovery from blood-brain barrier breakdown caused by brain injury

Glial activation is associated with cell proliferation and upregulation of astrocyte marker expression following traumatic injury in the brain. However, the biological significance of these processes remains unclear. In the present study, astrocyte activation was investigated in a murine brain injury model. Brain injury induces blood-brain barrier (BBB) breakdown and immunoglobulin G (IgG) leak into the brain parenchyma. The recovery of BBB breakdown was evaluated by analyzing immunofluorescent staining with mouse IgG antibody. IgG leakage was greatest at 1 day after stab wound injury and decreased thereafter, and almost diminished after 7 days. Bromodeoxy uridine incorporation was used, and astrocyte proliferation rates were examined by coimmunostaining with anti-bromodeoxy uridine and anti-glial fibrillary acid protein antibodies. Consistent with IgG leakage assays, astrocyte activation was the highest at day 3 and decreased after 7 days. Moreover, in reverse transcriptase-quantitative-PCR experiments, genes associated with BBB integrity were downregulated immediately after BBB breakdown and recovered to basal expression levels within 7 days. These data indicated that astrocyte activation correlated with BBB recovery from breakdown following brain injury.

Dissociation of blood-brain barrier disruption and disease manifestation in an aquaporin-4-deficient mouse model of amyotrophic lateral sclerosis

Aquaporin-4 (AQP4) is abundantly expressed in the central nervous system and is involved in the water balance in the cellular environment. Previous studies have reported that AQP4 expression is upregulated in rat models of amyotrophic lateral sclerosis (ALS), a fatal disease that affects motor neurons in the brain and spinal cord. In this study, we report that astrocytic AQP4 overexpression is evident during the course of disease in the spinal cord of an ALS mouse model, as well as in tissue from patients with ALS. AQP4 overexpression appears to be specifically associated with ALS because it was not induced by other experimental manipulations that produced acute or chronic gliosis. In order to examine the contribution of AQP4 to ALS disease development, we crossed AQP4 knockout mice with a mouse model of ALS. Significant improvement in blood-brain barrier (BBB) permeability was observed in the AQP4-deficient ALS mouse model. However, the time to disease onset and total lifespan were reduced in the AQP4-deficient ALS mouse model. The contradictory results suggest that changes in AQP4 may be context-dependent and further studies are required to understand the precise contribution of brain water balance in ALS.

Neuroimmunological Implications of AQP4 in Astrocytes

The brain has high-order functions and is composed of several kinds of cells, such as neurons and glial cells. It is becoming clear that many kinds of neurodegenerative diseases are more-or-less influenced by astrocytes, which are a type of glial cell. Aquaporin-4 (AQP4), a membrane-bound protein that regulates water permeability is a member of the aquaporin family of water channel proteins that is expressed in the endfeet of astrocytes in the central nervous system (CNS). Recently, AQP4 has been shown to function, not only as a water channel protein, but also as an adhesion molecule that is involved in cell migration and neuroexcitation, synaptic plasticity, and learning/memory through mechanisms involved in long-term potentiation or long-term depression. The most extensively examined role of AQP4 is its ability to act as a neuroimmunological inducer. Previously, we showed that AQP4 plays an important role in neuroimmunological functions in injured mouse brain in concert with the proinflammatory inducer osteopontin (OPN). The aim of this review is to summarize the functional implication of AQP4, focusing especially on its neuroimmunological roles. This review is a good opportunity to compile recent knowledge and could contribute to the therapeutic treatment of autoimmune diseases through strategies targeting AQP4. Finally, the author would like to hypothesize on AQP4’s role in interaction between reactive astrocytes and reactive microglial cells, which might occur in neurodegenerative diseases. Furthermore, a therapeutic strategy for AQP4-related neurodegenerative diseases is proposed.

A research update on the potential roles of aquaporin 4 in neuroinflammation

The presence of aquaporins (AQPs) in the brain has led to intense research on the underlying roles of this family of proteins under both normal and pathological conditions. Aquaporin 4 (AQP4) is the major water-channel membrane protein expressed in the central nervous system (CNS), primarily in astrocytes. Emerging evidence suggests that AQP4 could play an important role in water and ion homeostasis in the brain, and it has been studied in various brain pathological conditions. However, far less is known about the potential for AQP4 to influence neuroinflammation and, furthermore, its potential role in neurodegenerative disorders such as Alzheimer's disease (AD). It has been suggested that the pathogenesis of many clinical diseases, such as neuromyelitis optica (NMO), multiple sclerosis (MS) and brain injuries, is related to the regulation of AQP4 expression. Investigating the effects of AQP4 on microglia and astrocytes could be important to understand its role in the pathogenesis of neuroinflammation. Although the exact roles of non-steroidal anti-inflammatory drugs (NSAIDs) in protection against the detrimental effects of neuroinflammation remain unclear, research into the possible neuroprotective effects of AQP4 against neuroinflammation regulation seems to be important for future investigations.

The binding property of a monoclonal antibody against the extracellular domains of aquaporin-4 directs aquaporin-4 toward endocytosis

Neuromyelitis optica (NMO), an autoimmune disease of the central nervous system, is characterized by an autoantibody called NMO-IgG that recognizes the extracellular domains (ECDs) of aquaporin-4 (AQP4). In this study, monoclonal antibodies (mAbs) against the ECDs of mouse AQP4 were established by a baculovirus display method. Two types of mAb were obtained: one (E5415A) recognized both M1 and M23 isoforms, and the other (E5415B) almost exclusively recognized the square-array-formable M23 isoform. While E5415A enhanced endocytosis of both M1 and M23, followed by degradation in cells expressing AQP4, including astrocytes, E5415B did so to a much lesser degree, as determined by live imaging using fluorescence-labeled antibodies and by Western blotting of lysate of cells treated with these mAbs. E5415A promoted cluster formation of AQP4 on the cell surface prior to endocytosis as determined by immunofluorescent microscopic observation of bound mAbs to astrocytes as well as by Blue native PAGE analysis of AQP4 in the cells treated with the mAbs. These observations clearly indicate that an anti-AQP4-ECDs antibody possessing an ability to form a large cluster of AQP4 by cross-linking two or more tetramers outside the AQP4 arrays enhances endocytosis and the subsequent lysosomal degradation of AQP4.

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Two mAbs against the ECD of mAQP4 with different binding properties was established.

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One of them, E5415A, bound to mAQP4 independent of OAP-formation of AQP4.

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E5415A but not E5415B strongly enhanced endocytosis of endogenous AQP4 in astrocytes.

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E5415A formed large clusters of AQP4 cross-linking multiple AQP4 functional units.

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It is the cluster formation of AQP4 that triggers AQP4 endocytosis.

Microglia in central nervous system repair after injury

Accumulating evidence suggests that immune cells perform crucial inflammation-related functions including clearing dead tissue and promoting wound healing. Thus, they provide a conducive environment for better neuronal regeneration and functional recovery after adult mammalian central nervous system (CNS) injury. However, activated immune cells can also induce secondary damage of intact tissue and inhibit post-injury CNS repair. The inflammation response is due to the microglial production of cytokines and chemokines for the recruitment of peripheral immune cell populations, such as monocytes, neutrophils, dendritic cells and T lymphocytes. Interestingly, microglia and T lymphocytes can be detected at the injured site in both the early and later stages after nerve injury, whereas other peripheral immune cells infiltrate the injured parenchyma of the brain and spinal cord only in the early post-injury phase, and subsequently disappear. This suggests that microglia and T cells may play crucial roles in the post-injury functional recovery of the CNS. In this review, we summarize the current studies on microglia that examined neuronal regeneration and the molecular signalling mechanisms in the injured CNS. Better understanding of the effects of microglia on neural regeneration will aid the development of therapy strategies to enhance CNS functional recovery after injury.

Severely exacerbated neuromyelitis optica rat model with extensive astrocytopathy by high affinity anti-aquaporin-4 monoclonal antibody

Introduction

Neuromyelitis optica (NMO), an autoimmune astrocytopathic disease associated with anti-aquaporin-4 (AQP4) antibody, is characterized by extensive necrotic lesions preferentially involving the optic nerves and spinal cord. However, previous in-vivo experimental models injecting human anti-AQP4 antibodies only resulted in mild spinal cord lesions compared to NMO autopsied cases. Here, we investigated whether the formation of severe NMO-like lesions occurs in Lewis rats in the context of experimental autoimmune encephalomyelitis (EAE), intraperitoneally injecting incremental doses of purified human immunoglobulin-G from a NMO patient (hIgG_NMO) or a high affinity anti-AQP4 monoclonal antibody (E5415A), recognizing extracellular domain of AQP4 made by baculovirus display method.

Results

NMO-like lesions were observed in the spinal cord, brainstem, and optic chiasm of EAE-rats with injection of pathogenic IgG (hIgG_NMO and E5415A), but not in control EAE. Only in higher dose E5415A rats, there were acute and significantly severer clinical exacerbations (tetraparesis or moribund) compared with controls, within half day after the injection of pathogenic IgG. Loss of AQP4 was observed both in EAE rats receiving hIgG_NMO and E5415A in a dose dependent manner, but the ratio of AQP4 loss in spinal sections became significantly larger in those receiving high dose E5415A up to about 50 % than those receiving low-dose E5415A or hIgG_NMO less than 3 %. These lesions were also characterized by extensive loss of glial fibrillary acidic protein but relatively preserved myelin sheaths with perivascular deposition of IgG and C5b-9, which is compatible with post mortem NMO pathology. In high dose E5415A rats, massive neutrophil infiltration was observed especially at the lesion edge, and such lesions were highly vacuolated with partial demyelination and axonal damage. In contrast, such changes were absent in EAE rats receiving low-dose E5415A and hIgG_NMO.

Conclusions

In the present study, we established a severe experimental NMO rat model with highly clinical exacerbation and extensive tissue destructive lesions typically observed in NMO patients, which has not adequately been realized in in-vivo rodent models. Our data suggest that the pathogenic antibodies could induce immune mediated astrocytopathy with mobilized neutrophils, resulted in early lesion expansion of NMO lesion with vacuolation and other tissue damages. (350/350)

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0259-2) contains supplementary material, which is available to authorized users.

High avidity chimeric monoclonal antibodies against the extracellular domains of human aquaporin-4 competing with the neuromyelitis optica autoantibody, NMO-IgG

BACKGROUND AND PURPOSE

Most of the cases of neuromyelitis optica (NMO) are characterized by the presence of an autoantibody, NMO-IgG, which recognizes the extracellular domains of the water channel, aquaporin-4. Binding of NMO-IgG to aquaporin-4 expressed in end-feet of astrocytes leads to complement-dependent disruption of astrocytes followed by demyelination. One therapeutic option for NMO is to prevent the binding of NMO-IgG to aquaporin-4, using high-avidity, non-pathogenic-chimeric, monoclonal antibodies to this water channel. We describe here the development of such antibodies.

EXPERIMENTAL APPROACH

cDNAs encoding variable regions of heavy and light chains of monoclonal antibodies against the extracellular domains of human aquaporin-4 were cloned from hybridoma total RNA and fused to those encoding constant regions of human IgG1 and Igκ respectively. Then mammalian expression vectors were constructed to establish stable cell lines secreting mature chimeric antibodies.

KEY RESULTS

Original monoclonal antibodies showed high avidity binding to human aquaporin-4, as determined by ELISA. Live imaging using Alexa-Fluor-555-labelled antibodies revealed that the antibody D15107 more rapidly bound to cells expressing human aquaporin-4 than others and strongly enhanced endocytosis of this water channel, while D12092 also bound rapidly to human aquaporin-4 but enhanced endocytosis to a lesser degree. Chimeric D15107 prevented complement-dependent cytotoxicity induced by NMO-IgG from patient sera in vitro.

CONCLUSIONS AND IMPLICATIONS

We have established non-pathogenic, high-avidity, chimeric antibodies against the extracellular domains of human aquaporin-4, which provide a novel therapeutic option for preventing the progress and recurrence of NMO/NMO spectrum disorders.

The Potential Roles of Aquaporin 4 in Alzheimer's Disease

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system (CNS), and it is primarily expressed in astrocytes. It has been studied in various brain pathological conditions. However, the potential for AQP4 to influence Alzheimer's disease (AD) is still unclear. Research regarding AQP4 functions related to AD can be traced back several years and has gradually progressed toward a better understanding of the potential mechanisms. Currently, it has been suggested that AQP4 influences synaptic plasticity, and AQP4 deficiency may impair learning and memory, in part, through glutamate transporter-1 (GLT-1). AQP4 may mediate the clearance of amyloid beta peptides (Aβ). In addition, AQP4 may influence potassium (K(+)) and calcium (Ca(2+)) ion transport, which could play decisive roles in the pathogenesis of AD. Furthermore, AQP4 knockout is involved in neuroinflammation and interferes with AD. To date, no specific therapeutic agents have been developed to inhibit or enhance AQP4. However, experimental results strongly emphasize the importance of this topic for future investigations.

Aquaporin 4-dependent expression of glial fibrillary acidic protein and tenascin-C in activated astrocytes in stab wound mouse brain and in primary culture

We previously reported that aquaporin 4 (AQP4) has a neuroimmunological function via astrocytes and microglial cells involving osteopontin. AQP4 is a water channel localized in the endofoot of astrocytes in the brain, and its expression is upregulated after a stab wound to the mouse brain or the injection of methylmercury in common marmosets. In this study, the correlation between the expression of AQP4 and the expression of glial fibrillary acidic protein (GFAP) or tenascin-C (TN-C) in reactive astrocytes was examined in primary cultures and brain tissues of AQP4-deficient mice (AQP4/KO). In the absence of a stab wound to the brain or of any stimulation of the cells, the expressions of both GFAP and TN-C were lower in astrocytes from AQP4/KO mice than in those from wild-type (WT) mice. High levels of GFAP and TN-C expression were observed in activated astrocytes after a stab wound to the brain in WT mice; however, the expressions of GFAP and TN-C were insignificant in AQP4/KO mice. Furthermore, lipopolysaccharide (LPS) stimulation activated primary culture of astrocytes and upregulated GFAP and TN-C expression in cells from WT mice, whereas the expressions of GFAP and TN-C were slightly upregulated in cells from AQP4/KO mice. Moreover, the stimulation of primary culture of astrocytes with LPS also upregulated inflammatory cytokines in cells from WT mice, whereas modest increases were observed in cells from AQP4/KO mice. These results suggest that AQP4 expression accelerates GFAP and TN-C expression in activated astrocytes induced by a stab wound in the mouse brain and LPS-stimulated primary culture of astrocytes.

A novel monoclonal antibody against the C-terminal region of aquaporin-4

Aquaporin-4 (AQP4), the most abundant water channel in the brain, plays a central role in water homeostasis, neuronal activity, and migration of astrocytes in the central nervous system. Recent studies have demonstrated that AQP4 is a target of an autoantibody specifically detected in an autoimmune neurologic disease called neuromyelitis optica. Here we have generated a monoclonal antibody (MAb) against the C-terminal region of AQP4 using a baculovirus expressing mouse AQP4 as an immunogen. This antibody (clone E5206) recognized both human and mouse AQP4s in a denaturing condition and was able to precipitate AQP4 from cell lysates of CHO cells stably expressing AQP4. Western blot analysis using deletion mutants revealed that the epitope was located within a region between Asp(303) and Leu(320) in the C-terminal tail of AQP4. Although clone E5206 could not be used for immunostaining when cells or tissues were fixed with 4% paraformaldehyde or 10% formalin, it could be used when cells were fixed with 10% trichloroacetic acid and when a formalin-fixed tissue section was pretreated with antigen-retrieval reagents. This MAb can be a valuable tool for analysis of AQP4 in a variety of physiological and pathophysiological contexts, in human tissues and organs as well as in rodent models, both in vitro and in vivo.

Unprecedented cell-selection using ultra-quick freezing combined with aquaporin expression

Freezing is usually used for preservation and storage of biological samples; however, this process may have some adverse effects such as cell membrane damage. Aquaporin (AQP), a water channel protein, has been suggested to play some roles for cryopreservation although its molecular mechanism remains unclear. Here we show that membrane damage caused by ultra-quick freezing is rescued by the expression of AQP4. We next examine if the expression of AQP combined with ultra-quick freezing can be used to select cells efficiently under freezing conditions where most cells are died. CHO cells stably expressing AQP4 were exclusively selected from mixed cell cultures. Having identified the increased expression of AQP4 during ES cell differentiation into neuro-ectoderm using bioinformatics, we confirmed the improved survival of differentiated ES cells with AQP4 expression. Finally we show that CHO cells transiently transfected with Endothelin receptor A and Aqp4 were also selected and concentrated by multiple cycles of freezing/thawing, which was confirmed with calcium imaging in response to endothelin. Furthermore, we found that the expression of AQP enables a reduction in the amount of cryoprotectants for freezing, thereby decreasing osmotic stress and cellular toxicity. Taken together, we propose that this simple but efficient and safe method may be applicable to the selection of mammalian cells for applications in regenerative medicine as well as cell-based functional assays or drug screening protocols.

Neuroinflammation and central nervous system regeneration in vertebrates

Injuries in the central nervous system (CNS) are one of the leading causes of mortality or persistent disabilities in humans. One of the reasons why humans cannot recover from neuronal loss is the limited regenerative capacity of their CNS. By contrast, non-mammalian vertebrates exhibit widespread regeneration in diverse tissues including the CNS. Understanding those mechanisms activated during regeneration may improve the regenerative outcome in the severed mammalian CNS. Of those mechanisms, recent evidence suggests that inflammation may be important in regeneration. In this review we compare the different events following acute CNS injury in mammals and non-mammalian vertebrates. We also discuss the involvement of the immune response in initiating regenerative programs and how immune cells and neural stem/progenitor cells (NSPCs) communicate.

AQP5 is differentially regulated in astrocytes during metabolic and traumatic injuries

Water movement plays vital roles in both physiological and pathological conditions in the brain. Astrocytes are responsible for regulating this water movement and are the major contributors to brain edema in pathological conditions. Aquaporins (AQPs) in astrocytes play critical roles in the regulation of water movement in the brain. AQP1, 3, 4, 5, 8, and 9 have been reported in the brain. Compared with AQP1, 4, and 9, AQP3, 5, and 8 are less studied. Among the lesser known AQPs, AQP5, which has multiple functions identified outside the central nervous system, is also indicated to be involved in hypoxia injury in astrocytes. In our study, AQP5 expression could be detected both in primary cultures of astrocytes and neurons, and AQP5 expression in astrocytes was confirmed in 1- to 4-week old primary cultures of astrocytes. AQP5 was localized on the cytoplasmic membrane and in the cytoplasm of astrocytes. AQP5 expression was downregulated during ischemia treatment and upregulated after scratch-wound injury, which was also confirmed in a middle cerebral artery occlusion model and a stab-wound injury model in vivo. The AQP5 increased after scratch injury was polarized to the migrating processes and cytoplasmic membrane of astrocytes in the leading edge of the scratch-wound, and AQP5 over-expression facilitated astrocyte process elongation after scratch injury. Taken together, these results indicate that AQP5 might be an important water channel in astrocytes that is differentially expressed during various brain injuries.