Hyaluronan synthesis supports glutamate transporter activity

Hyaluronan is synthesized, secreted, and anchored by hyaluronan synthases (HAS) at the plasma membrane and comprises the backbone of perineuronal nets around neuronal soma and dendrites. However, the molecular targets of hyaluronan to regulate synaptic transmission in the central nervous system have not been fully identified. Here, we report that hyaluronan is a negative regulator of excitatory signals. At excitatory synapses, glutamate is removed by glutamate transporters to turn off the signal and prevent excitotoxicity. Hyaluronan synthesized by HAS supports the activity of glial glutamate transporter 1 (GLT1). GLT1 also retracted from cellular processes of cultured astrocytes after hyaluronidase treatment and hyaluronan synthesis inhibition. A serial knockout study showed that all three HAS subtypes recruit GLT1 to cellular processes. Furthermore, hyaluronidase treatment activated neurons in a dissociated rat hippocampal culture and caused neuronal damage due to excitotoxicity. Our findings reveal that hyaluronan helps to turn off excitatory signals by supporting glutamate clearance. Cover Image for this issue: doi: 10.1111/jnc.14516.

[Epileptogenesis mediated by glial cells]

Epilepsy is one of the most common diseases of the central nervous system. Many epilepsies are controllable because of the existence of different antiepileptic drugs with multiple mechanisms of action. However, about 30% of epilepsy is so-called refractory epilepsy in which existing drugs do not show enough therapeutic effects. Antiepileptic drugs can be roughly divided into two types, i.e., those that suppress the excitability of neuronal cells and those that promote inhibition. Inhibition of excitatory neurons include a variety of ion channel inhibitors such as Na⁺, drugs that inhibit glutamate release and glutamate AMPA receptor, whereas enhancement of inhibitory neurons includes a drug that enhances GABA_A receptor. Both are targeted to neurons. Recent advances in brain science have revealed the importance of the role of glial cells in regulation of brain function and excitability of neurons. Although glia cells themselves are electrically non-excitable cells, they could greatly affect excitability of neurons by controlling extracellular neurotransmitters, glial transmitters, regulating various ions concentration, regulation of energy metabolism, and formation/elimination of synapses. Therefore, when the function of glial cells changes, these regulatory functions also change, which in turn greatly changes the excitability of neurons and neuronal networks. Epilegenicity is a condition in which the brain is likely to undergo spontaneous epileptic seizures and it is suggested that modulation of the above-mentioned glial cell function is greatly related to the acquisition of epileptogenesis. In this article, I focus on astrocytes among glial cells, and describe the relationship between functional modulation and epileptogenesis when changing to the phenotype of reactive astrocytes by epileptic seizures. We also discuss development of antiepileptic drugs targeting reactive astrocytes.

Microglia mediate non-cell-autonomous cell death of retinal ganglion cells

Excitotoxicity is well known in the neuronal death in the brain and is also linked to neuronal damages in the retina. Recent accumulating evidence show that microglia greatly affect excitotoxicity in the brain, but their roles in retina have received only limited attention. Here, we report that retinal excitotoxicity is mediated by microglia. To this end, we employed three discrete methods, that is, pharmacological inhibition of microglia by minocycline, pharmacological ablation by an antagonist for colony stimulating factor 1 receptor (PLX5622), and genetic ablation of microglia using Iba1-tTA::DTA^tetO/tetO mice. Intravitreal injection of NMDA increased the number of apoptotic retinal ganglion cells (RGCs) followed by reduction in the number of RGCs. Although microglia did not respond to NMDA directly, they became reactive earlier than RGC damages. Inhibition or ablation of microglia protected RGCs against NMDA. We found up-regulation of proinflammatory cytokine genes including Il1b, Il6 and Tnfa, among which Tnfa was selectively blocked by minocycline. PLX5622 also suppressed Tnfa expression. Tumor necrosis factor α (TNFα) signals were restricted in microglia at very early followed by spreading into other cell types. TNFα up-regulation in microglia and other cells were significantly attenuated by minocycline and PLX5622, suggesting a central role of microglia for TNFα induction. Both inhibition of TNFα and knockdown of TNF receptor type 1 by siRNA protected RGCs against NMDA. Taken together, our data demonstrate that a phenotypic change of microglia into a neurotoxic one is a critical event for the NMDA-induced degeneration of RGCs, suggesting an importance of non-cell-autonomous mechanism in the retinal neuronal excitotoxicity.

Involvement of VNUT-exocytosis in transient receptor potential vanilloid 4-dependent ATP release from gastrointestinal epithelium

Adenosine triphosphate (ATP) modulates mechanosensitive vagal afferent nerves in the gastrointestinal tract. ATP is stored in secretory vesicles via the ATP transporter VNUT. Recently, the bisphosphate clodronate was reported to inhibit VNUT and was suggested to be a safe potent therapeutic option for chronic pain. Transient receptor potential vanilloid 4 (TRPV4) is activated by mechanical stimuli and some epoxyeicosatrienoic acids and becomes sensitized under inflammatory conditions. We have previously reported that TRPV4 and VNUT are expressed in mouse esophageal keratinocytes and that TRPV4 activation induces ATP release in gastric epithelial cells. Here we show the expression of TRPV4 and VNUT in normal human gastrointestinal cell derived cell lines (GES-1 and CCD 841) and in tissues from normal and VNUT-KO mice. TRPV4 agonists (GSK101 or 8,9-EET) induced an increase in cytosolic Ca²⁺ and/or current responses in mouse primary colonic epithelial cells and CCD 841 cells, but not in cells isolated from TRPV4-KO mice. TRPV4 agonists (GSK101 or 5.6-EET) also induced ATP release in GES-1 and CCD 841 cells, which could be blocked by the VNUT inhibitor, clodronate. Thus, VNUT inhibition with clodronate could represent a novel therapeutic option for visceral pain.

The time-dependent variation of ATP release in mouse primary-cultured urothelial cells is regulated by the clock gene

AIMS

The sensation of bladder fullness (SBF) is triggered by the release of ATP. Therefore, the aim of this study was to investigate whether time-dependent changes in the levels of stretch-released ATP in mouse primary-cultured urothelial cells (MPCUCs) is regulated by circadian rhythm via clock genes.

METHODS

MPCUCs were derived from wild-type and Clock mutant mice (Clock^Δ19/Δ19 ), presenting a nocturia phenotype. They were cultured in elastic silicone chambers. Stretch-released ATP was quantified every 4 h by ATP photon count. An experiment was also performed to determine whether ATP release correlated with the rhythm of the expression of Piezo1, TRPV4, VNUT, and Connexin26 (Cx26) in MPCUCs regulated by clock genes with circadian rhythms. MPCUCs were treated with carbenoxolone, an inhibitor of gap junction protein; were derived from VNUT-KO mice; or treated with Piezo1-siRNA, TRPV4-siRNA, and Cx26-siRNA.

RESULTS

Stretch-released ATP showed time-dependent changes in wild-type mice and correlated with the rhythm of the expression of Piezo1, TRPV4, VNUT, and Cx26. However, these rhythms were disrupted in Clock^Δ19/Δ19 mice. Carbenoxolone eliminated the rhythmicity of ATP release in wild-type mice. However, time-dependent ATP release changes were maintained when a single gene was deficient such as VNUT-KO, Piezo1-, TRPV4-, and Cx26-siRNA.

CONCLUSIONS

ATP release in the bladder urothelium induces SBF and may have a circadian rhythm regulated by the clock genes. In the bladder urothelium, clock gene abnormalities may disrupt circadian ATP release by inducing Piezo1, TRPV4, VNUT, and Cx26. All these genes can trigger nocturia.

Anti-Depressant Fluoxetine Reveals its Therapeutic Effect Via Astrocytes

Although psychotropic drugs act on neurons and glial cells, how glia respond, and whether glial responses are involved in therapeutic effects are poorly understood. Here, we show that fluoxetine (FLX), an anti-depressant, mediates its anti-depressive effect by increasing the gliotransmission of ATP. FLX increased ATP exocytosis via vesicular nucleotide transporter (VNUT). FLX-induced anti-depressive behavior was decreased in astrocyte-selective VNUT-knockout mice or when VNUT was deleted in mice, but it was increased when astrocyte-selective VNUT was overexpressed in mice. This suggests that VNUT-dependent astrocytic ATP exocytosis has a critical role in the therapeutic effect of FLX. Released ATP and its metabolite adenosine act on P2Y₁₁ and adenosine A2b receptors expressed by astrocytes, causing an increase in brain-derived neurotrophic factor in astrocytes. These findings suggest that in addition to neurons, FLX acts on astrocytes and mediates its therapeutic effects by increasing ATP gliotransmission.

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Anti-depressant FLX acts on astrocytes and increases VNUT-dependent ATP exocytosis.

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Such astrocytic responses are responsible for the FLX-induced therapeutic effects.

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Astrocytic ATP and its metabolite adenosine increase BDNF in astrocytes, and reveal the therapeutic effects.

Kinoshita et al. demonstrated that astrocytes are a therapeutic target of the antidepressant, fluoxetine (FLX). They found that FLX stimulates VNUT-dependent ATP release from astrocytes leading to a BDNF-mediated anti-depressive effect. This study demonstrated the astrocytic regulation of this anti-depressive effect, which complements the previously described conventional mechanism of FLX. Because the involvement of astrocytes in the pathogenesis of depression is of current interest, this new insight into the role of astrocytes in anti-depressive effects should support the establishment of novel therapeutic strategies for depression.

The oscillation of intracellular Ca2+ influx associated with the circadian expression of Piezo1 and TRPV4 in the bladder urothelium

We previously showed that bladder functions are controlled by clock genes with circadian rhythm. The sensation of bladder fullness (SBF) is sensed by mechano-sensor such as Piezo1 and TRPV4 in the mouse bladder urothelium. However, functional circadian rhythms of such mechano-sensors remain unknown. To investigate functional circadian changes of these mechano-sensors, we measured circadian changes in stretch-evoked intracellular Ca²⁺ influx ([Ca²⁺] _i ) using mouse primary cultured urothelial cells (MPCUCs). Using Ca²⁺ imaging, stretch-evoked [Ca²⁺] _i was quantified every 4 h in MPCUCs derived from wild-type (WT) and Clock ^Δ19/Δ19 mice, which showed a nocturia phenotype. Furthermore, a Piezo1 inhibitor GsMTx4 and a TRPV4 inhibitor Ruthenium Red were applied and stretch-evoked [Ca²⁺] _i in MPCUCs was measured to investigate their contribution to SBF. Stretch-evoked [Ca²⁺] _i showed a circadian rhythm in the WT mice. In contrast, Clock ^Δ19/Δ19 mice showed disrupted circadian rhythm. The administration of both GsMTx4 and Ruthenium Red eliminated the circadian rhythm of stretch-evoked [Ca²⁺] _i in WT mice. We conclude that SBF may have a circadian rhythm, which is created by functional circadian changes of Piezo1 and TRPV4 being controlled by clock genes to be active during wakefulness and inactive during sleep. Abnormalities of clock genes disrupt SBF, and induce nocturia.

New roles of reactive astrocytes in the brain; an organizer of cerebral ischemia

The brain consists of neurons and much higher number of glial cells. They communicate each other, by which they control brain functions. The brain is highly vulnerable to several insults such as ischemia, but has a self-protective and self-repairing mechanisms against these. Ischemic tolerance or preconditioning is an endogenous neuroprotective phenomenon, where a mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Glial cells are structurally in close association with synapses. Recent studies have uncovered the active roles of astrocytes in modulating synaptic connectivity, such as synapse formation, elimination and maturation, during development or pathology. However, glia-mediated ischemic tolerance and/or neuronal repairing have received only limited attention. We and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in regulation of induction of ischemic tolerance as well as repairing/remodeling of neuronal networks by phagocytosis. Here, we review our current understanding of (1) glial-mediated ischemic tolerance and (2) glia-mediated repairing/remodeling of the penumbra neuronal networks, and highlight their mechanisms as well as their potential benefits, problems, and therapeutic application.

Aberrant astrocyte Ca2+ signals "AxCa signals" exacerbate pathological alterations in an Alexander disease model

Alexander disease (AxD) is a rare neurodegenerative disorder caused by gain of function mutations in the glial fibrillary acidic protein (GFAP) gene. Accumulation of GFAP proteins and formation of Rosenthal fibers (RFs) in astrocytes are hallmarks of AxD. However, malfunction of astrocytes in the AxD brain is poorly understood. Here, we show aberrant Ca²⁺ responses in astrocytes as playing a causative role in AxD. Transcriptome analysis of astrocytes from a model of AxD showed age-dependent upregulation of GFAP, several markers for neurotoxic reactive astrocytes, and downregulation of Ca²⁺ homeostasis molecules. In situ AxD model astrocytes produced aberrant extra-large Ca²⁺ signals "AxCa signals", which increased with age, correlated with GFAP upregulation, and were dependent on stored Ca²⁺ . Inhibition of AxCa signals by deletion of inositol 1,4,5-trisphosphate type 2 receptors (IP3R2) ameliorated AxD pathogenesis. Taken together, AxCa signals in the model astrocytes would contribute to AxD pathogenesis.

Expression of Periostin in Patients with Non-Small Cell Lung Cancer: Correlation with Angiogenesis and Lymphangiogenesis

The aim of this study was to evaluate periostin expression measured immunohistochemically in patients with non-small cell lung cancer (NSCLC) and to determine its association with clinical features, prognosis, angiogenesis, and lymphangiogenesis. We investigated periostin expression in a series of 88 patients with NSCLC. We also determined whether expression of periostin correlated with microvessel density and lymphatic microvessel density. Periostin was expressed in 42% of 88 patients. Its expression was significantly correlated with tumor size, lymph node metastasis, disease stage, and lymphatic invasion (p=0.0128, 0.0015, 0.0310 and 0.0273, respectively). There also was a significant relation between periostin expression and microvessel density and lymphatic microvessel density (all p<0.0001). Five-year survival rates were better in patients with negative periostin expression than in those with positive periostin expression (p=0.0044). Periostin expression was not significant in a multivariate additive model. Our findings show that periostin correlates with increased tumor progression and a worse prognosis in NSCLC, as well as with angiogenesis and lymphangiogenesis.

Purinergic dysregulation causes hypertensive glaucoma-like optic neuropathy

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs) and visual loss. Although one of the highest risk factors for glaucoma is elevated intraocular pressure (IOP) and reduction in IOP is the only proven treatment, the mechanism of IOP regulation is poorly understood. We report that the P2Y6 receptor is critical for lowering IOP and that ablation of the P2Y6 gene in mice (P2Y6KO) results in hypertensive glaucoma-like optic neuropathy. Topically applied uridine diphosphate, an endogenous selective agonist for the P2Y6 receptor, decreases IOP. The P2Y6 receptor was expressed in nonpigmented epithelial cells of the ciliary body and controlled aqueous humor dynamics. P2Y6KO mice exhibited sustained elevation of IOP, age-dependent damage to the optic nerve, thinning of ganglion cell plus inner plexiform layers, and a reduction of RGC numbers. These changes in P2Y6KO mice were attenuated by an IOP lowering agent. Consistent with RGC damage, visual functions were impaired in middle-aged P2Y6KO mice. We also found that expression and function of P2Y6 receptors in WT mice were significantly reduced by aging, another important risk factor for glaucoma. In summary, our data show that dysfunctional purinergic signaling causes IOP dysregulation, resulting in glaucomatous optic neuropathy.

An effective therapeutic approach for oxaliplatin-induced peripheral neuropathy using a combination therapy with goshajinkigan and bushi

Oxaliplatin-induced peripheral neuropathy (OIPN) occurs at extraordinarily high frequency, but no effective treatment for this disorder has been established. Goshajinkigan (GJG), a traditional Japanese medicine known as Kampo, is known to reduce OIPN in both basic and clinical studies. However, its molecular mechanisms remain largely unknown. Here, we elucidate the mechanisms underlying the therapeutic effects of GJG against OIPN and the therapeutic benefits of combining GJG with bushi, a herbal medicine derived from the processed Aconiti tuber. Oxaliplatin (4 mg/kg) was injected into mice twice a week for up to 4 and 3 weeks, respectively. OIPN was assessed using pain behavioral tests, such as those testing cold hypersensitivity, thermal hyperalgesia, and mechanical allodynia, as well as a reduction of the current perception threshold (CPT). GJG (0.3 or 1 g/kg) and bushi (0.1 or 0.3 g/kg) were orally administered 5 times a week for 4 weeks. Behavioral analysis was performed 24 h after the final dose.

Oxaliplatin induced cold hypersensitivity and mechanical allodynia but not thermal hyperalgesia and reduced CPT of Aδ- and Aβ-fibers but not C-fibers. All these effects were counteracted by GJG. Bushi, an ingredient of GJG that shows analgesic effect, reduced oxaliplatin-induced cold hypersensitivity but had no effect on oxaliplatin-induced mechanical allodynia. However, bushi significantly accentuated the effects of GJG when co-administered with GJG. GJG reduces OIPN by counteracting the sensitization of Aδ- and Aβ-fibers and shows analgesic effects against cold hypersensitivity and mechanical allodynia. These effects are potentiated by bushi. The combination of GJG with bushi has high potential for preventing OIPN.

Pathologic Active mTOR Mutation in Brain Malformation with Intractable Epilepsy Leads to Cell-Autonomous Migration Delay

The activation of phosphatidylinositol 3-kinase-AKTs-mammalian target of rapamycin cell signaling pathway leads to cell overgrowth and abnormal migration and results in various types of cortical malformations, such as hemimegalencephaly (HME), focal cortical dysplasia, and tuberous sclerosis complex. However, the pathomechanism underlying abnormal cell migration remains unknown. With the use of fetal mouse brain, we performed causative gene analysis of the resected brain tissues from a patient with HME and investigated the pathogenesis. We obtained a novel somatic mutation of the MTOR gene, having approximately 11% and 7% mutation frequency in the resected brain tissues. Moreover, we revealed that the MTOR mutation resulted in hyperphosphorylation of its downstream molecules, S6 and 4E-binding protein 1, and delayed cell migration on the radial glial fiber and did not affect other cells. We suspect cell-autonomous migration arrest on the radial glial foot by the active MTOR mutation and offer potential explanations for why this may lead to cortical malformations such as HME.

Polymorphic regulation of mitochondrial fission and fusion modifies phenotypes of microglia in neuroinflammation

Microglia are the resident macrophages of the central nervous system and play complex roles in the milieu of diseases including the primary diseases of myelin. Although mitochondria are critical for cellular functions and survival in the nervous system, alterations in and the roles of mitochondrial dynamics and associated signaling in microglia are still poorly understood. In the present study, by combining immunohistochemistry and 3D ultrastructural analyses, we show that mitochondrial fission/fusion in reactive microglia is differentially regulated from that in monocyte-derived macrophages and the ramified microglia of normal white matter in myelin disease models. Mouse cerebral microglia in vitro demonstrated that stimulation of TLR4 with lipopolysaccharide, widely used to examine microglial reactions, caused the activation of the mitochondrial fission protein, dynamin-related protein 1 (Drp1) and enhanced production of reactive oxygen species (ROS). The increase in the ROS level activated 5' adenosine monophosphate-activated protein kinase (AMPK), and facilitated elongation of mitochondria along the microtubule tracks. These results suggest that the polymorphic regulation of mitochondrial fission and fusion in reactive microglia is mediated by distinct signaling under inflammatory conditions, and modulates microglial phenotypes through the production of ROS.

Reactive astrocytes function as phagocytes after brain ischemia via ABCA1-mediated pathway

Astrocytes become reactive following various brain insults; however, the functions of reactive astrocytes are poorly understood. Here, we show that reactive astrocytes function as phagocytes after transient ischemic injury and appear in a limited spatiotemporal pattern. Following transient brain ischemia, phagocytic astrocytes are observed within the ischemic penumbra region during the later stage of ischemia. However, phagocytic microglia are mainly observed within the ischemic core region during the earlier stage of ischemia. Phagocytic astrocytes upregulate ABCA1 and its pathway molecules, MEGF10 and GULP1, which are required for phagocytosis, and upregulation of ABCA1 alone is sufficient for enhancement of phagocytosis in vitro. Disrupting ABCA1 in reactive astrocytes result in fewer phagocytic inclusions after ischemia. Together, these findings suggest that astrocytes are transformed into a phagocytic phenotype as a result of increase in ABCA1 and its pathway molecules and contribute to remodeling of damaged tissues and penumbra networks.

Astrocytic phagocytosis has been shown to play a role in synaptic pruning during development, but whether adult astrocytes possess phagocytic ability is unclear. Here the authors show that following brain ischemia, reactive astrocytes become phagocytic and engulf debris via the ABCA1 pathway.

Astrocyte-mediated synapse remodeling in the pathological brain

Astrocytes, a major type of glia, reciprocally influence synaptic transmission and connectivity, forming the "tripartite synapses". Astrocytic metabotropic glutamate receptor (mGluR)-mediated Ca²⁺ waves and release of gliotransmitters or synaptogenic molecules mediate this neuron-glia interaction in the developing brain, but this signaling has been challenged for adult brain. However, cumulative evidence has suggested that mature astrocytes exhibit re-awakening of such immature phenotype in the pathological adult brain. This phenotypic change in astrocytes in response to injury may induce neural circuit and synapse plasticity. In this review article, we summarize astrocyte-mediated synapse remodeling during physiological development, discuss re-emergence of immature astrocytic signaling in adult pathological brain, and finally highlight its contribution to significant modification of synaptic connections correlating with functional progress of brain pathology.

Carbenoxolone inhibits TRPV4 channel-initiated oxidative urothelial injury and ameliorates cyclophosphamide-induced bladder dysfunction

Carbenoxolone (CBX) is a clinically prescribed drug for the treatment of digestive ulcer and inflammation. It is also a widely used pharmacological inhibitor of several channels in basic research. Given that the overactivity of several channels, including those inhibitable by CBX, underlies bladder dysfunction, we tested the potential therapeutic application and mechanism of CBX in the treatment of voiding dysfunction. In a mouse model of cystitis induced by cyclophosphamide (CYP), CBX administration prevented the CYP‐elicited increase in bladder weight, oedema, haemorrhage, and urothelial injury. CBX also greatly improved micturition pattern, as manifested by the apparently decreased micturition frequency and increased micturition volume. Western blot results showed that CBX suppressed CYP‐induced increase in protein carbonyls, COX‐2, and iNOS. Further analysis using cultured urothelial cells revealed that acrolein, the major metabolite of CYP, caused protein oxidation, p38 activation, and urothelial injury. These effects of acrolein were reproduced by TRPV4 agonists and significantly prevented by antioxidant NAC, p38 inhibitor SB203580, TRPV4 antagonist RN‐1734, and CBX. Further studies showed that CBX potently suppressed TRPV4 agonist‐initiated calcium influx and subsequent cell injury. CBX attenuated CYP‐induced cystitis in vivo and reduced acrolein‐induced cell injury in vitro, through mechanisms involving inhibition of TRPV4 channels and attenuation of the channel‐mediated oxidative stress. CBX might be a promising agent for the treatment of bladder dysfunction.

Hypoxia-independent mechanisms of HIF-1α expression in astrocytes after ischemic preconditioning

We recently demonstrated that ischemic tolerance was dependent on astrocytes, for which HIF-1α had an essential role. The mild ischemia (preconditioning; PC) increased HIF-1α in a biphasic pattern, that is, a quick and transient increase in neurons, followed by a slow and sustained increase in astrocytes. However, mechanisms underlying such temporal difference in HIF-1α increase remain totally unknown. Here, we show that unlike a hypoxia-dependent mechanism in neurons, astrocytes increase HIF-1α via a novel hypoxia-independent but P2X7-dependent mechanism. Using a middle cerebral artery occlusion (MCAO) model of mice, we found that the PC (a 15-min MCAO period)-evoked increase in HIF-1α in neurons was quick and transient (from 1 to 3 days after PC), but that in astrocytes was slow-onset and long-lasting (from 3 days to at least 2 weeks after PC). The neuronal HIF-1α increase was dependent on inhibition of PHD2, an oxygen-dependent HIF-1α degrading enzyme, whereas astrocytic one was independent of PHD2. Astrocytes even do not possess this enzyme. Instead, they produced a sustained increase in P2X7 receptors, activation of which resulted in HIF-1α increase. The hypoxia-independent but P2X7-receptor-dependent mechanism could allow astrocytes to cause long-lasting HIF-1α expression, thereby leading to induction of ischemic tolerance efficiently. GLIA 2017;65:523-530.