Glial Cell Function Research Articles

Activity-dependent functions of non-electrical glial cells.

Electrical activity is essential for brain function. However, neurons, the electrically active cells, are less numerous than the non-electrical glial cells in the central nervous system. The non-electrical components modify the function of neural circuits, depending on the electrical neuronal activity, by wrapping synapses, myelinating axons and phagocytozing the neuronal components. Moreover, recent evidence has suggested that they contribute to neurological and psychiatric disease by regulating neuronal circuits, ultimately affecting their behaviour. In this review, we highlight the physiological functions of glial cells, particularly the electrical activity-dependent processes, to provide further insight into their role in brain function.

Glial Regulation of Energy Metabolism.

The major function of brain glial cells is to maintain a homeostatic milieu for neurons to work properly in response to a variety of environmental alterations. Recent studies have shown that glial cells in the hypothalamus, a brain center controlling homeostatic physiological functions, are essential for regulating energy metabolism in both physiological and pathological conditions. Astrocytes, tanycytes, and NG2-glia shuttle and/or sense key metabolic factors presented to the hypothalamus either directly, by glial metabolic enzymes, receptors, and transporters, or indirectly, by modulating the sensing ability of other types of hypothalamic cells. Astrocytes, tanycytes, and microglia are critically important in the development and maintenance of hypothalamic circuits regulating energy balance. Hypothalamic inflammation commonly associated with diet-induced obesity is manifested via hypothalamic reactive gliosis involving microglia and astrocytes, contributing to the correlated abnormal energy metabolism. Although many glial functions in energy metabolism remain to be fully elucidated, we are at the dawn of targeting glia-neuron interactions in the hypothalamus for therapeutic applications in metabolic disorders.

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 GABAA 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.

Neuroglia: A New Open-Access Journal Publishing All Aspects of Glial Research

Today, we announce the new journal Neuroglia, which we see as an inclusive and innovative open-access forum for publishing all aspects of glial research.[...]

Imaging the neuroimmune response to alcohol exposure in adolescent baboons: a TSPO PET study using 18 F-DPA-714.

The effects of acute alcohol exposure to the central nervous system are hypothesized to involve the innate immune system. The neuroimmune response to an initial and acute alcohol exposure was investigated using translocator protein 18kDa (TSPO) PET imaging, a non-invasive marker of glial activation, in adolescent baboons. Three different alcohol-naive adolescent baboons (3-4years old, 9 to 14kg) underwent 18 F-DPA-714 PET experiments before, during and 7-12months after this initial alcohol exposure (0.7-1.0g/l). The brain distribution of 18 F-DPA-714 (VT ; in ml/cm3 ) was estimated in several brain regions using the Logan plot analysis and the metabolite-corrected arterial input function. Compared with alcohol-naive animals (VTbrain =3.7±0.7ml/cm3 ), the regional VT s of 18 F-DPA-714 were significantly increased during alcohol exposure (VTbrain =7.2±0.4ml/cm3 ; p<0.001). Regional VT s estimated several months after alcohol exposure (VTbrain =5.7±1.4ml/cm3 ) were lower (p<0.001) than those measured during alcohol exposure, but remained significantly higher (p<0.001) than in alcohol-naive animals. The acute and long-term effects of ethanol exposure were observed globally across all brain regions. Acute alcohol exposure increased the binding of 18 F-DPA-714 to the brain in a non-human primate model of alcohol exposure that reflects the 'binge drinking' situation in adolescent individuals. The effect persisted for several months, suggesting a 'priming' of glial cell function after initial alcohol exposure.

Potential role of sirtuin 1 in Müller glial cells in mice choroidal neovascularization.

This study investigated the potential role of sirtuin 1 in Müller glial cells in choroidal neovascularization. In the in vitro study, primary Müller glial cells were cultured and treated with resveratrol, a sirtuin 1 activator. Glial fibrillary acidic protein expression and angiogenesis-related gene expression were examined using quantitative polymerase chain reaction and phagocytosis, as a marker of Müller glial cell function; in addition, a latex bead assay was used to analyze cell function. For the in vivo study, choroidal neovascularization was induced in C57BL/6 mice via laser photocoagulation, and resveratrol was administered intravitreally. Eyecup whole mounts were created to measure choroidal neovascularization volumes on day 7. Immunohistochemical analysis with anti-glial fibrillary acidic protein antibody was used to detect Müller glial cell activation in eyes with choroidal neovascularization on day 1, 3, 5, and 7 after laser surgery. Resveratrol significantly promoted glial fibrillary acidic protein, anti-angiogenic factor, pigment epithelium-derived factor, and thrombospondin-1 expression in the cells as well as the phagocytic activities. Treatment of the choroidal neovascularization model with resveratrol resulted in early activation of Müller glial cells near choroidal neovascularization sites. Resveratrol-activated cells but not the controls migrated to the top of choroidal neovascularization sites and into the lesions from day 3. Resveratrol reduced the choroidal neovascularization size relative to controls. In conclusion, sirtuin 1 activation in Müller glial cells suppressed the development of choroidal neovascularization, and therefore, might be a therapeutic option.

Differential modulation of glial cell mediated neuroinflammation in Plasmodium berghei ANKA infection by TGF β and IL 6

Modulation of pro-inflammatory and anti-inflammatory axis and orientation of glial cell function towards neuroinflammation, were hallmark signs of cerebral malaria (CM). CM pathogenesis was concerned with the circulating levels of Interleukin 6 (IL 6) and Transforming growth factor β (TGF β). Definite roles of these two cytokines in brain related pathology remained largely unexplored. To study the effect of these two cytokines, we have examined changes in morphology and in activation profile of the glial cells after TGF β and IL 6 neutralization during CM in cortex and cerebellum of the Plasmodium berghei ANKA (PbA) infected male swiss albino mice. PbA infection caused severe inflammation by inducing changes in morphological features as well as in activation profile of the astrocytes and microglia. Similar inflammatory signs were evident in Anti TGF β treated set. Interestingly in the Anti IL 6 treated set, reduced level of activation of these glial cells corresponds to the reduced level of inflammatory profile. Microglial activation was found to be synchronous with TLR4 engagement. Neuronal death was triggered by neuroinflammatory milieu seen in PbA and PbA+Anti TGF β treated set. In conclusion, it can be said that IL 6 and TGF β perform essential role in CM pathogenesis by modulating the level of glial cell induced neuroinflammation.

Enteric glial–mediated enhancement of intestinal barrier integrity is compromised by morphine

BackgroundThe opioid epidemic is a growing concern, and emerging evidence suggests that morphine use may be associated with sepsis. Enteric glial cells (EGCs) are the most numerous cell type in the enteric nervous system and regulate gastrointestinal function through the production of trophic factors, including glial-derived neurotrophic factor (GDNF). We sought to determine the effect of morphine on enteric glia and hypothesized that morphine contributes to EGC dysfunction and increased gut permeability. Materials and methodsRat intestinal epithelial cells (IECs) and EGC lines were purchased from ATCC. Immunocytochemistry was used to evaluate the impact of EGCs on IEC barrier proteins and detect the μ-opioid receptor. Co-culture assays were used to determine the effect of EGCs, GDNF, and morphine on barrier integrity. Quantitative polymerase chain reaction and western blotting were performed to determine the impact of morphine in GDNF production. Transepithelial resistance of IEC-6 cell monolayers was measured in the presence of EGC-conditioned media (EGC-CM) and morphine treated EGC-CM using electrical cell impedance sensing. ResultsEGC-CM enhanced tight junction organization in IECs. IEC barrier integrity was enhanced when co-cultured with unstimulated EGCs or with GDNF alone; this barrier protective effect was lost with morphine-treated EGCs. GDNF RNA and protein expression were decreased by morphine treatment. Transepithelial resistance was decreased in IEC confluent monolayers when exposed to morphine-treated EGC-CM compared with control. ConclusionsMorphine compromises intestinal epithelial cell barrier function through a mechanism which appears to involve GDNF. Further studies are warranted to delineate the role of enteric glial cell function in opioid signaling and sepsis.

Mitochondrial dysfunction in glial cells: Implications for neuronal homeostasis and survival

Mitochondrial dysfunction is central to the pathogenesis of neurological disorders. Neurons rely on oxidative phosphorylation to meet their energy requirements and thus alterations in mitochondrial function are linked to energy failure and neuronal cell death. Furthermore, in neurons, dysfunctional mitochondria are reported to increase the steady-state levels of reactive oxygen species derived from the leakage of electrons from the electron transport chain. Research aimed at understanding mitochondrial dysfunction and its role in neurological disorders has been primarily geared towards neurons. In contrast, the effects of mitochondrial dysfunction in glial cells’ function and its implication for neuronal homeostasis and brain function has been largely understudied. Unlike neurons and oligodendrocytes, astrocytes and microglia do not degenerate upon the impairment of mitochondrial function, as they rely primarily on glycolysis to produce energy and have a higher antioxidant capacity than neurons. However, recent evidence highlights the role of mitochondrial metabolism and signaling in glial cell function. In this work, we review the functional role of mitochondria in glial cells and the evidence regarding its potential role regulating neuronal homeostasis and disease progression.

A Lifetime\u2019s Adventure in Extracellular K+ Regulation: The Scottish Connection

In a career that has spanned 45 years and shows no signs of slowing down, Dr Bruce Ransom has devoted considerable time and energy to studying regulation of interstitial K+. When Bruce commenced his studies in 1969 virtually nothing was known of the functions of glial cells, but Bruce’s research contributed to the physiological assignation of function to mammalian astrocytes, namely interstitial K+ buffering. The experiments that I describe in this review concern the response of the membrane potential (Em) of in vivo cat cortical astrocytes to changes in [K+]o, an experimental manoeuvre that was achieved in two different ways. The first involved recording the Em of an astrocyte while the initial aCSF was switched to one with different K+, whereas in the second series of experiments the cortex was stimulated and the response of the astrocyte Em to the K+ released from neighbouring neurons was recorded. The astrocytes responded in a qualitatively predictable manner, but quantitatively the changes were not as predicted by the Nernst equation. Elevations in interstitial K+ are not sustained and K+ returns to baseline rapidly due to the buffering capacity of astrocytes, a phenomenon studied by Bruce, and his son Chris, published 27 years after Bruce’s initial publications. Thus, a lifetime spent investigating K+ buffering has seen enormous advances in glial research, from the time cells were identified as ‘presumed’ glial cells or ‘silent cells’, to the present day, where glial cells are recognised as contributing to every important physiological brain function.

719 - Corticotropin-Releasing Hormone Receptor 2 (CRHR2) Mediates Enteric Glial Cell Function During Colitis

719 - Corticotropin-Releasing Hormone Receptor 2 (CRHR2) Mediates Enteric Glial Cell Function During Colitis

Activation of FXR pathway does not alter glial cell function

BackgroundThe nuclear receptor farnesoid-X-receptor (FXR; NR1H4) is expressed not only in the liver, gut, kidney and adipose tissue but also in the immune cells. FXR has been shown to confer protection in several animal models of inflammation, including experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). FXR agonists are currently tested in clinical trials for treatment of human metabolic diseases. The beneficial effect of FXR agonists in EAE suggests that FXR might represent a potential target in inflammatory-demyelinating CNS diseases, such as MS. In MS, oligodendrocytes not only undergo cell death but also contribute to remyelination. This repair mechanism is impaired due to a differentiation block of oligodendroglial progenitor cells. Activation of other nuclear receptors that heterodimerize with FXR promote oligodendroglial differentiation. Therefore, we wanted to address the functional relevance of FXR for glial cells, especially for oligodendroglial differentiation.MethodsWe isolated primary murine oligodendrocytes from FXR-deficient (FXR Ko) and wild-type (WT) mice and determined the effect of FXR deficiency and activation on oligodendroglial differentiation by analysing markers of oligodendroglial progenitor cells (OPCs) and mature oligodendrocytes (OLs) using qRT-PCR and immunocytochemistry. Additionally, we determined whether FXR activation modulates the pro-inflammatory profile of astrocytes or microglia and whether this may subsequently modulate oligodendroglial differentiation. These in vitro studies were complemented by histological analyses of oligodendrocytes in FXR Ko mice.ResultsFXR is expressed by OPCs and mature oligodendrocytes. However, lack of FXR did not affect oligodendroglial differentiation in vitro or in vivo. Furthermore, activation of FXR using the synthetic agonist GW4064 did not affect oligodendroglial differentiation, remyelination in an ex vivo model or the expression of pro-inflammatory molecules in astrocytes or microglia. Concordantly, no effects of supernatants from macrophages cultured in the presence of GW4064 were observed regarding a possible indirect impact on oligodendroglial differentiation.ConclusionsOur data suggest that FXR is dispensable for oligodendroglial differentiation and that FXR agonists, such as GW4064, represent a potential therapeutic approach for MS which specifically targets peripheral immune cells including macrophages but not brain-resident cells, such as oligodendrocytes, astrocytes or microglia.

TRPM2 contributes to LPC-induced intracellular Ca2+ influx and microglial activation

Microglia are the resident immune cells which become activated in some pathological conditions in central nervous system (CNS). Lysophosphatidylcholine (LPC), an endogenous inflammatory phospholipid, is implicated in immunomodulatory function of glial cells in the CNS. Although several studies uncovered that LPC induces intracellular Ca2+ influx and morphologic change in microglia, there is still no direct evidence showing change of phosphorylation of mitogen-activated protein kinase (MAPK) p38 (p-p38), a widely used microglia activation marker, by LPC. Furthermore, the cellular mechanism of LPC-induced microglia activation remains unknown. In this study, we found that LPC induced intracellular Ca2+ increase in primary cultured microglia, which was blocked in the presence of Gd3+, non-selective transient receptor potential (TRP) channel blocker. RT-PCR and whole cell patch clamp recordings revealed molecular and functional expression of TRP melastatin 2 (TRPM2) in microglia. Using western blotting, we also observed that LPC increased phosphorylation of p38 MAPK, and the increase of p-p38 expression is also reversed in TRPM2-knockout (KO) microglia. Moreover, LPC induced membrane trafficking of TRPM2 and intrathecal injection of LPC increased Iba-1 immunoreactivity in the spinal cord, which were significantly reduced in KO mice. In addition, LPC-induced intracellular Ca2+ increase and inward currents were abolished in TRPM2-KO microglia. Taken together, our results suggest that LPC induces intracellular Ca2+ influx and increases phosphorylation of p38 MAPK via TRPM2, which in turn activates microglia.

Glial Cells and Their Function in the Adult Brain: A Journey through the History of Their Ablation.

Glial cells, consisting of microglia, astrocytes, and oligodendrocyte lineage cells as their major components, constitute a large fraction of the mammalian brain. Originally considered as purely non-functional glue for neurons, decades of research have highlighted the importance as well as further functions of glial cells. Although many aspects of these cells are well characterized nowadays, the functions of the different glial populations in the brain under both physiological and pathological conditions remain, at least to a certain extent, unresolved. To tackle these important questions, a broad range of depletion approaches have been developed in which microglia, astrocytes, or oligodendrocyte lineage cells (i.e., NG2-glia and oligodendrocytes) are specifically ablated from the adult brain network with a subsequent analysis of the consequences. As the different glial populations are very heterogeneous, it is imperative to specifically ablate single cell populations instead of inducing cell death in all glial cells in general. Thanks to modern genetic manipulation methods, the approaches can now directly be targeted to the cell type of interest making the ablation more specific compared to general cell ablation approaches that have been used earlier on. In this review, we will give a detailed summary on different glial ablation studies, focusing on the adult mouse central nervous system and the functional readouts. We will also provide an outlook on how these approaches could be further exploited in the future.

Aquaporins in peripheral nociception

Aquaporins in peripheral nociception

Pleiotropic function of TRPV4 ion channels in the central nervous system.

What is the topic of this review? In this concise review, we highlight insights into the role of transient receptor potential, vanilloid type 4 (TRPV4) ion channels in the CNS, results that have been contributed over the last 16 years since the initial discovery of the channel. What advances does it highlight? TRPV4 has been found to function in neurons, astroglia and microglia, both in physiological (e.g. astrocytic neurovascular coupling, neuronal membrane potential at physiological temperature) and in pathological conditions (e.g. mechanical trauma), so far recorded as exciting findings in need of more in-depth mechanistic clarification. Transient receptor potential, vanilloid type 4 (TRPV4) ion channels are osmo-mechano-TRP channels, with pleiotropic function and expression in many different types of tissues and cells. They have also been found to be involved in pain and inflammation. Studies have focused on the role of TRPV4 in peripheral sensory neurons, but its expression and function in central nervous glial cells and neurons has also been documented. In this overview, based on the senior author's (WL) lecture at the recent recent joint meeting of APS/The Physiological Society in Dublin, we concisely review evidence of TRPV4 expression and function in the CNS and how TRPV4 function can be modulated for therapeutic benefit of neuropsychiatric disorders. Novel TRPV4-inhibitory compounds developed recently in the authors' laboratory are also discussed.

Protease-Activated Receptor 2: Are Common Functions in Glial and Immune Cells Linked to Inflammation-Related CNS Disorders?

Protease-activated receptors (PARs) are a novel family of G-protein coupled receptors (GPCRs) whose activation requires the cleavage of the N-terminus by a serine protease. However, recent evidence reveals that alternative routes of activation also occur, that PARs signal via multiple pathways and that pathway activation is activator- dependent. Given our increased understanding of PAR function both under physiological and pathophysiological conditions, one aspect that has remained constant is the link between PAR2 and inflammation. PAR2 is expressed in immune cells of both the innate and adaptive immune system and has been shown to play a role in several peripheral inflammatory conditions. PAR2 is similarly expressed on astrocytes and microglia within the CNS and its activation is either protective or detrimental to CNS function depending on the conditions or disease state investigated. With a clear similarity between the function of PAR2 on both immune cells and CNS glial cells, here we have reviewed their roles in both these systems. We suggest that the recent development of novel PAR2 modulators, including those that show biased signalling, will further increase our understanding of PAR2 function and the development of potential therapeutics for CNS disorders in which inflammation is proposed to play a role.

Regulation of neuronal axon specification by glia-neuron gap junctions in C. elegans.

Axon specification is a critical step in neuronal development, and the function of glial cells in this process is not fully understood. Here, we show that C. elegans GLR glial cells regulate axon specification of their nearby GABAergic RME neurons through GLR-RME gap junctions. Disruption of GLR-RME gap junctions causes misaccumulation of axonal markers in non-axonal neurites of RME neurons and converts microtubules in those neurites to form an axon-like assembly. We further uncover that GLR-RME gap junctions regulate RME axon specification through activation of the CDK-5 pathway in a calcium-dependent manner, involving a calpain clp-4. Therefore, our study reveals the function of glia-neuron gap junctions in neuronal axon specification and shows that calcium originated from glial cells can regulate neuronal intracellular pathways through gap junctions.

Sex differences in astrocyte and microglia responses immediately following middle cerebral artery occlusion in adult mice

Epidemiological studies report that infarct size is decreased and stroke outcomes are improved in young females when compared to males. However, mechanistic insight is lacking. We posit that sex-specific differences in glial cell functions occurring immediately after ischemic stroke are a source of dichotomous outcomes. In this study we assessed astrocyte Ca2+ dynamics, aquaporin 4 (AQP4) polarity, S100β expression pattern, as well as, microglia morphology and phagocytic marker CD11b in male and female mice following 60min of middle cerebral artery (MCA) occlusion. We reveal sex differences in the frequency of intracellular astrocyte Ca2+ elevations (F(1,86)=8.19, P=0.005) and microglia volume (F(1,40)=12.47, P=0.009) immediately following MCA occlusion in acute brain slices. Measured in fixed tissue, AQP4 polarity was disrupted (F(5,86)=3.30, P=0.009) and the area of non-S100β immunoreactivity increased in ipsilateral brain regions after 60min of MCA occlusion (F(5,86)=4.72, P=0.007). However, astrocyte changes were robust in male mice when compared to females. Additional sex differences were discovered regarding microglia phagocytic receptor CD11b. In sham mice, constitutively high CD11b immunofluorescence was observed in females when compared to males (P=0.03). When compared to sham, only male mice exhibited an increase in CD11b immunoreactivity after MCA occlusion (P=0.006). We posit that a sex difference in the presence of constitutive CD11b has a role in determining male and female microglia phagocytic responses to ischemia. Taken together, these findings are critical to understanding potential sex differences in glial physiology as well as stroke pathobiology which are foundational for the development of future sex-specific stroke therapies.

Far beyond the motor neuron: the role of glial cells in amyotrophic lateral sclerosis.

Motor neuron disease is one of the major groups of neurodegenerative diseases, mainly represented by amyotrophic lateral sclerosis. Despite wide genetic and biochemical data regarding its pathophysiological mechanisms, motor neuron disease develops under a complex network of mechanisms not restricted to the unique functions of the alpha motor neurons but which actually involve diverse functions of glial cell interaction. This review aims to expose some of the leading roles of glial cells in the physiological mechanisms of neuron-glial cell interactions and the mechanisms related to motor neuron survival linked to glial cell functions.