K+ modulation of microglial superoxide production: involvement of voltage-gated Ca2+ channels

Nimodipine inhibits spreading depolarization, ischemic injury, and neuroinflammation in mouse live brain slice preparations

Nimodipine is used to prevent delayed ischemic deficit in patients with aneurysmal subarachnoid hemorrhage (aSAH). Spreading depolarization (SD) is recognized as a factor in the pathomechanism of aSAH and other acute brain injuries. Although nimodipine is primarily known as a cerebral vasodilator, it may have a more complex mechanism of action due to the expression of its target, the L-type voltage-gated calcium channels (LVGCCs) in various cells in neural tissue. This study was designed to investigate the direct effect of nimodipine on SD, ischemic tissue injury, and neuroinflammation. SD in control or nimodipine-treated live mouse brain slices was induced under physiological conditions using electrical stimulation, or by subjecting the slices to hypo-osmotic stress or mild oxygen-glucose deprivation (mOGD). SD was recorded applying local field potential recording or intrinsic optical signal imaging. Histological analysis was used to estimate tissue injury, the number of reactive astrocytes, and the degree of microglia activation. Nimodipine did not prevent SD occurrence in mOGD, but it did reduce the rate of SD propagation and the cortical area affected by SD. In contrast, nimodipine blocked SD occurrence in hypo-osmotic stress, but had no effect on SD propagation. Furthermore, nimodipine prevented ischemic injury associated with SD in mOGD. Nimodipine also exhibited anti-inflammatory effects in mOGD by reducing reactive astrogliosis and microglial activation. The results demonstrate that nimodipine directly inhibits SD, independent of nimodipine's vascular effects. Therefore, the use of nimodipine may be extended to treat acute brain injuries where SD plays a central role in injury progression.

The role of intracellular calcium-store-mediated calcium signals in in vivo sensor and effector functions of microglia

Under physiological conditions microglia, the immune sentinels of the brain, constantly monitor their microenvironment. In the case of danger, damage or cell/tissue dyshomeostasis, they react with changes in process motility, polarization, directed process movement, morphology and gene expression profile; release pro- and anti-inflammatory mediators; proliferate; and clean brain parenchyma by means of phagocytosis. Based on recent transcriptomic and in vivo Ca2+ imaging data, we argue that the local cell/tissue dyshomeostasis is sensed by microglia via intracellular Ca2+ signals, many of which are mediated by Ca2+ release from the intracellular Ca2+ stores. These signals encode the strength, duration and spatiotemporal pattern of the stimulus and, at the same time, relay this information further to trigger the respective Ca2+ -dependent effector pathways. We also point to the fact that microglial Ca2+ signalling is sexually dimorphic and undergoes profound changes across the organism's lifespan. Interestingly, the first changes in microglial Ca2+ signalling are visible already in 9- to 11-month-old mice, roughly corresponding to 40-year-old humans.

L-type calcium channel antagonist isradipine age-dependently decreases plaque associated dystrophic neurites in 5XFAD mouse model

Epidemiological studies suggest that L-type calcium channel (LTCC) antagonists may reduce the incidence of age-associated neurodegenerative diseases including Alzheimer's disease (AD). However, the neuroprotective mechanism of LTCC antagonists is unknown. Amyloid-β (Aβ) pathology disrupts intracellular calcium signaling, which regulates lysosomes and microglial responses. Neurons near Aβ plaques develop dystrophic neurites, which are abnormal swellings that accumulate lysosomes. Further, microglia accumulate around Aβ plaques and secrete inflammatory cytokines. We hypothesized that antagonism of LTCCs with isradipine would reduce Aβ plaque-associated dystrophic neurites and inflammatory microglia in the 5XFAD mouse model by restoring normal intracellular calcium regulation. To test this hypothesis, we treated 6- and 9-month-old 5XFAD mice with isradipine and tested behavior, examined Aβ plaques, microglia, and dystrophic neurites. We found that isradipine treatment age-dependently reduces dystrophic neurites and leads to trending decreases in Aβ but does not modulate plaque associated microglia regardless of age. Our findings provide insight into how antagonizing LTCCs alters specific cell types in the Aβ plaque environment, providing valuable information for potential treatment targets in future AD studies.

Electrified microglia: Impact of direct current stimulation on diverse properties of the most versatile brain cell

BACKGROUND

Transcranial direct current stimulation [(t)DCS], modulates cortical excitability and promotes neuroplasticity. Microglia has been identified to respond to electrical currents as well as neuronal activity, but its response to DCS is mostly unknown.

OBJECTIVE

This study addresses effects of DCS applied in vivo to the sensorimotor cortex on physiological microglia properties and neuron-microglia communication.

METHODS

Time lapse in vivo 2-photon microscopy in anaesthetized mice was timely coupled with DCS of the sensorimotor cortex to observe microglia dynamics on a population-based and single cell level. Neuron-microglia communication during DCS was investigated in mice with a functional knock out of the fractalkine receptor CX3CR1. Moreover, the role of voltage gated microglial channels and DCS effects on phagocytosis were studied.

RESULTS

DCS promoted several physiological microglia properties, depending on the glial activation state and stimulation intensity. On a single cell level, process motility was predominantly enhanced in ramified cells whereas horizontal soma movement and galvanotaxis was pronounced in reactive microglia. Blockage of voltage sensitive microglial channels suppressed DCS effects in vivo and in vitro. Microglial motility changes were partially driven by the fractalkine signaling pathway. Moreover, phagocytosis increased after DCS in vitro.

CONCLUSION

Microglia dynamics are rapidly influenced by DCS. This is the first in vivo demonstration of a direct effect of electrical currents on microglia and indirect effects potentially driven by neuronal activity via the fractalkine pathway.

Sex differences in neuro(auto)immunity and chronic sciatic nerve pain

Chronic pain occurs with greater frequency in women, with a parallel sexually dimorphic trend reported in sufferers of many autoimmune diseases. There is a need to continue examining neuro-immune-endocrine crosstalk in the context of sexual dimorphisms in chronic pain. Several phenomena in particular need to be further explored. In patients, autoantibodies to neural antigens have been associated with sensory pathway hyper-excitability, and the role of self-antigens released by damaged nerves remains to be defined. In addition, specific immune cells release pro-nociceptive cytokines that directly influence neural firing, while T lymphocytes activated by specific antigens secrete factors that either support nerve repair or exacerbate the damage. Modulating specific immune cell populations could therefore be a means to promote nerve recovery, with sex-specific outcomes. Understanding biological sex differences that maintain, or fail to maintain, neuroimmune homeostasis may inform the selection of sex-specific treatment regimens, improving chronic pain management by rebalancing neuroimmune feedback. Given the significance of interactions between nerves and immune cells in the generation and maintenance of neuropathic pain, this review focuses on sex differences and possible links with persistent autoimmune activity using sciatica as an example.

Strategies for Neuroprotection in Multiple Sclerosis and the Role of Calcium

Calcium ions are vital for maintaining the physiological and biochemical processes inside cells. The central nervous system (CNS) is particularly dependent on calcium homeostasis and its dysregulation has been associated with several neurodegenerative disorders including Parkinson’s disease (PD), Alzheimer’s disease (AD) and Huntington’s disease (HD), as well as with multiple sclerosis (MS). Hence, the modulation of calcium influx into the cells and the targeting of calcium-mediated signaling pathways may present a promising therapeutic approach for these diseases. This review provides an overview on calcium channels in neurons and glial cells. Special emphasis is put on MS, a chronic autoimmune disease of the CNS. While the initial relapsing-remitting stage of MS can be treated effectively with immune modulatory and immunosuppressive drugs, the subsequent progressive stage has remained largely untreatable. Here we summarize several approaches that have been and are currently being tested for their neuroprotective capacities in MS and we discuss which role calcium could play in this regard.

Targeting microglia L-type voltage-dependent calcium channels for the treatment of central nervous system disorders

Calcium (Ca²⁺ ) is a ubiquitous mediator of a multitude of cellular functions in the central nervous system (CNS). Intracellular Ca²⁺ is tightly regulated by cells, including entry via plasma membrane Ca²⁺ permeable channels. Of specific interest for this review are L-type voltage-dependent Ca²⁺ channels (L-VDCCs), due to their pleiotropic role in several CNS disorders. Currently, there are numerous approved drugs that target L-VDCCs, including dihydropyridines. These drugs are safe and effective for the treatment of humans with cardiovascular disease and may also confer neuroprotection. Here, we review the potential of L-VDCCs as a target for the treatment of CNS disorders with a focus on microglia L-VDCCs. Microglia, the resident immune cells of the brain, have attracted recent attention for their emerging inflammatory role in several CNS diseases. Intracellular Ca²⁺ regulates microglia transition from a resting quiescent state to an "activated" immune-effector state and is thus a valuable target for manipulation of microglia phenotype. We will review the literature on L-VDCC expression and function in the CNS and on microglia in vitro and in vivo and explore the therapeutic landscape of L-VDCC-targeting agents at present and future challenges in the context of Alzheimer's disease, Parkinson's disease, Huntington's disease, neuropsychiatric diseases, and other CNS disorders.

The Protective Effect of Omeprazole Against Traumatic Brain Injury: An Experimental Study

BACKGROUND

The development of secondary brain injury via oxidative stress after traumatic brain injury (TBI) is a well-known entity. Consequently, the aim of the present study was to evaluate the role of omeprazole (OM) on rat model of TBI.

METHODS

A total of 24 male rats were used and divided into 4 groups as follows; control, trauma, OM, and methylprednisolone (MP). The trauma, OM, and MP groups were subjected to closed-head contusive weight-drop injuries. Rats received treatment with saline, OM, or MP, respectively. All the animals were sacrificed at 24 hours after trauma and brain tissues were extracted. The oxidant/antioxidant parameters (malondialdehyde, glutathione peroxidase, superoxide dismutase, nitric oxide) and caspase-3 in the cerebral tissue were analyzed, and histomorphologic evaluation of the cerebral tissue was performed.

RESULTS

Levels of MDA and activity of caspase-3 were significantly reduced in the OM and MP groups compared with the trauma group. Glutathione peroxidase and superoxide dismutase levels were increased both in the OM and MP groups compared with the trauma group. The pathology scores were statistically lower in the OM and MP groups than the trauma group.

CONCLUSIONS

The results of the present study showed that OM was as effective as MP in protecting brain from oxidative stress, and apoptosis in the early phase of TBI.

Estrogen ameliorates microglial activation by inhibiting the Kir2.1 inward-rectifier K(+) channel

Microglial activation is implicated in the pathogenesis of Parkinson's disease (PD). Although the etiology of PD remains unclear, age and male gender are known PD risk factors. By comparing microglia and dopaminergic (DA) neurons in the substantia nigra (SN) of male and female mice of different ages, we found that the degrees of microglial activation and DA neuron loss increased with age in both genders, but were more pronounced in males, as were peripheral lipopolysaccharide (LPS)-induced microglial activation and DA neuron loss. A bilateral ovariectomy (OVX) eliminated the female-associated protection against age- and LPS-induced microglial activation, which suggests that ovary hormones are involved in gender-specific responses. Treating female mice with 17β-estradiol supplements reduced the age-associated microglial activation in OVX mice. Moreover, pretreating mouse BV2 microglial cells with 17β-estradiol inhibited LPS-induced elevation of Toll-like receptor 4, phosphorylated p38, and TNF-α levels. We then examined the effect of 17β-estradiol on inward-rectifier K(+) channel Kir2.1, a known regulator of microglial activation. We found that 17β-estradiol inhibited the Kir2.1 activity of BV2 cells by reducing the probability that the channel would be open. We conclude that age- and inflammation-associated microglial activation is attenuated by ovarian estrogen, because it inhibits Kir2.1.

Calcium dysregulation via L-type voltage-dependent calcium channels and ryanodine receptors underlies memory deficits and synaptic dysfunction during chronic neuroinflammation

Background

Chronic neuroinflammation and calcium (Ca⁺²) dysregulation are both components of Alzheimer’s disease. Prolonged neuroinflammation produces elevation of pro-inflammatory cytokines and reactive oxygen species which can alter neuronal Ca⁺² homeostasis via L-type voltage-dependent Ca⁺² channels (L-VDCCs) and ryanodine receptors (RyRs). Chronic neuroinflammation also leads to deficits in spatial memory, which may be related to Ca⁺² dysregulation.

Methods

The studies herein use an in vivo model of chronic neuroinflammation: rats were infused intraventricularly with a continuous small dose of lipopolysaccharide (LPS) or artificial cerebrospinal fluid (aCSF) for 28 days. The rats were treated with the L-VDCC antagonist nimodipine or the RyR antagonist dantrolene.

Results

LPS-infused rats had significant memory deficits in the Morris water maze, and this deficit was ameliorated by treatment with nimodipine. Synaptosomes from LPS-infused rats had increased Ca⁺² uptake, which was reduced by a blockade of L-VDCCs either in vivo or ex vivo.

Conclusions

Taken together, these data indicate that Ca⁺² dysregulation during chronic neuroinflammation is partially dependent on increases in L-VDCC function. However, blockade of the RyRs also slightly improved spatial memory of the LPS-infused rats, demonstrating that other Ca⁺² channels are dysregulated during chronic neuroinflammation. Ca⁺²-dependent immediate early gene expression was reduced in LPS-infused rats treated with dantrolene or nimodipine, indicating normalized synaptic function that may underlie improvements in spatial memory. Pro-inflammatory markers are also reduced in LPS-infused rats treated with either drug. Overall, these data suggest that Ca⁺² dysregulation via L-VDCCs and RyRs play a crucial role in memory deficits resulting from chronic neuroinflammation.

The L-type voltage-gated calcium channel modulates microglial pro-inflammatory activity

Under pathological conditions, microglia, the resident CNS immune cells, become reactive and release pro-inflammatory cytokines and neurotoxic factors. We investigated whether this phenotypic switch includes changes in the expression of the L-type voltage-gated calcium channel (VGCC) in a rat model of N-methyl-D-aspartate-induced hippocampal neurodegeneration. Double immunohistochemistry and confocal microscopy evidenced that activated microglia express the L-type VGCC. We then analyzed whether BV2 microglia express functional L-type VGCC, and investigated the latter's role in microglial cytokine release and phagocytic capacity. Activated BV2 microglia express the CaV1.2 and CaV1.3 subunits of the L-type VGCC determined by reverse transcription-polymerase chain reaction, Western blot and immunocytochemistry. Depolarization with KCl induced a Ca2+ entry facilitated by Bay k8644 and partially blocked with nifedipine, which also reduced TNF-α and NO release by 40%. However, no nifedipine effect on BV2 microglia viability or phagocytic capacity was observed. Our results suggest that in CNS inflammatory processes, the L-type VGCC plays a specific role in the control of microglial secretory activity.

Differential rescue of spatial memory deficits in aged rats by L-type voltage-dependent calcium channel and ryanodine receptor antagonism

Age-associated memory impairments may result as a consequence of neuroinflammatory induction of intracellular calcium (Ca(+2)) dysregulation. Altered L-type voltage-dependent calcium channel (L-VDCC) and ryanodine receptor (RyR) activity may underlie age-associated learning and memory impairments. Various neuroinflammatory markers are associated with increased activity of both L-VDCCs and RyRs, and increased neuroinflammation is associated with normal aging. In vitro, pharmacological blockade of L-VDCCs and RyRs has been shown to be anti-inflammatory. Here, we examined whether pharmacological blockade of L-VDCCs or RyRs with the drugs nimodipine and dantrolene, respectively, could improve spatial memory and reduce age-associated increases in microglia activation. Dantrolene and nimodipine differentially attenuated age-associated spatial memory deficits but were not anti-inflammatory in vivo. Furthermore, RyR gene expression was inversely correlated with spatial memory, highlighting the central role of Ca(+2) dysregulation in age-associated memory deficits.

Calcium ion influx in microglial cells: physiological and therapeutic significance

Microglial cells, the immunocompetent cells of the central nervous system (CNS), exhibit a resting phenotype under healthy conditions. In response to injury, however, they transform into an activated state, which is a hallmark feature of many CNS diseases. Factors or agents released from the neurons, blood vessels, and/or astrocytes could activate these cells, leading to their functional and structural modifications. Microglial cells are well equipped to sense environmental changes within the brain under both physiological and pathological conditions. Entry of calcium ions (Ca(2+)) plays a critical role in the process of microglial transformation; several channels and receptors have been identified on the surface of microglial cells. These include store-operated channel, Orai1, and its sensor protein, stromal interaction molecule 1 (STIM1), in microglial cells, and their functions are modulated under pathological stimulations. Transient receptor potential (TRP) channels and voltage- and ligand-gated channels (ionotropic and metabotropic receptors) are also responsible for Ca(2+) influx into the microglial cells. An elevation of intracellular Ca(2+) concentration subsequently regulates microglial cell functions by activating a diverse array of Ca(2+)-sensitive signaling cascades. Perturbed Ca(2+) homeostasis contributes to the progression of a number of CNS disorders. Thus, regulation of Ca(2+) entry into microglial cells could be a pharmacological target for several CNS-related pathological conditions. This Review addresses the recent insights into microglial cell Ca(2+) influx mechanisms, their roles in the regulation of functions, and alterations of Ca(2+) entry in specific CNS disorders.

Microglial ion channels as potential targets for neuroprotection in Parkinson's disease

Parkinson's disease (PD) is a chronic, degenerative neurological disorder that is estimated to affect at least 1 million individuals in the USA and over 10 million worldwide. It is thought that the loss of neurons and development of inclusion bodies occur gradually over decades until they progress to the point where ~60% of the dopamine neurons are lost and patients present with motor dysfunction. At present, it is not clear what causes this progression, and there are no current therapies that have been successful in preventing PD progression. Although there are many hypotheses regarding the mechanism of PD progression, neuroinflammation may be a major contributor to PD pathogenesis. Indeed, activated microglia and subsequent neuroinflammation have been consistently associated with the pathogenesis of PD. Thus, interference with this process could provide a means of neuroprotection in PD. This review will discuss the potential of targeting microglia to reduce neuroinflammation in PD. Further, we discuss the potential of microglial ion channels to serve as novel targets for neuroprotection in PD.

Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity

Microglia are resident cells of the brain involved in regulatory processes critical for development, maintenance of the neural environment, injury and repair. They belong to the monocytic-macrophage lineage and serve as brain immune cells to orchestrate innate immune responses; however, they are distinct from other tissue macrophages due to their relatively quiescent phenotype and tight regulation by the CNS microenvironment. Microglia actively survey the surrounding parenchyma and respond rapidly to changes such that any disruption to neural architecture or function can contribute to the loss in regulation of the microglia phenotype. In many models of neurodegeneration and neurotoxicity, early events of synaptic degeneration and neuronal loss are accompanied by an inflammatory response including activation of microglia, perivascular monocytes, and recruitment of leukocytes. In culture, microglia have been shown to be capable of releasing several potentially cytotoxic substances, such as reactive oxygen intermediates, nitric oxide, proteases, arachidonic acid derivatives, excitatory amino acids, and cytokines; however, they also produce various neurotrophic factors and quench damage from free radicals and excitotoxins. As the primary source for pro-inflammatory cytokines, microglia are implicated as pivotal mediators of neuroinflammation and can induce or modulate a broad spectrum of cellular responses. Neuroinflammation should be considered as a balanced network of processes whereby subtle modifications can shift the cells toward disparate outcomes. For any evaluation of neuroinflammation and microglial responses, within the framework of neurotoxicity or degeneration, one key question in determining the consequence of neuroinflammation is whether the response is an initiating event or the consequence of tissue damage. As examples of environmental exposure-related neuroinflammation in the literature, we provide an evaluation of data on manganese and diesel exhaust particles.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Microglial calcium signal acts as a rapid sensor of single neuron damage in vivo

In the healthy adult brain microglia, the main immune-competent cells of the CNS, have a distinct (so-called resting or surveying) phenotype. Resting microglia can only be studied in vivo since any isolation of brain tissue inevitably triggers microglial activation. Here we used in vivo two-photon imaging to obtain a first insight into Ca(2+) signaling in resting cortical microglia. The majority (80%) of microglial cells showed no spontaneous Ca(2+) transients at rest and in conditions of strong neuronal activity. However, they reliably responded with large, generalized Ca(2+) transients to damage of an individual neuron. These damage-induced responses had a short latency (0.4-4s) and were localized to the immediate vicinity of the damaged neuron (< 50 μm cell body-to-cell body distance). They were occluded by the application of ATPγS as well as UDP and 2-MeSADP, the agonists of metabotropic P2Y receptors, and they required Ca(2+) release from the intracellular Ca(2+) stores. Thus, our in vivo data suggest that microglial Ca(2+) signals occur mostly under pathological conditions and identify a Ca(2+) store-operated signal, which represents a very sensitive, rapid, and highly localized response of microglial cells to brain damage. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Nimodipine protects dopaminergic neurons against inflammation-mediated degeneration through inhibition of microglial activation

Nimodipine, a calcium channel blocker, has been used mainly in the therapy of cardiovascular diseases. Recently, its indications have been extended experimentally to a wider range of disorders especially some central nervous system (CNS) disorders. In this study, we investigated whether nimodipine is neuroprotective to inflammation-mediated neurodegenerative diseases. Pretreatment with nimodipine reduced the degeneration of dopaminergic (DA) neurons induced by LPS in mesencephalic neuron-glia cultures in a dose-dependent manner. The neuroprotective effect of nimodipine was attributed to the inhibition of microglial activation, since nimodipine significantly inhibited the production of nitric oxide (NO), tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and prostaglandin E(2) (PGE(2)) from LPS-stimulated microglia. Moreover, nimodipine was not neuroprotective to 1-methyi-4-phenylpyridinium (MPP(+))-induced DA neurotoxicity in the absence of microglia. Mechanistic study showed that nimodipine failed to protect the degeneration of neurons in neuron-glia cultures from mice lacking functional NADPH oxidase (PHOX), a key enzyme for extracellular superoxide production in immune cells. Taken together these results suggest that nimodipine is protective to DA neurodegeneration via inhibiting the microglial-mediated oxidative stress and inflammatory response. Thus, nimodipine may be a potential therapeutic agent for the treatment of inflammation-related neurodegenerative disorders such as Parkinson's disease.

Calcium and glial cell death

Calcium (Ca2+) homeostasis is crucial for development and survival of virtually all types of cells including glia of the central nervous system (CNS). Astrocytes, oligodendrocytes and microglia, the major glial cell types in the CNS, are endowed with a rather sophisticated array of Ca2+-permeable receptors and channels, as well as store-operated channels and pumps, all of which determine Ca2+ homeostasis. In addition, glial cells detect functional activity in neighbouring neurons and respond to it by means of Ca2+ signals that can modulate synaptic interactions. Like in neurons, Ca2+ overload resulting from dysregulation of channels and pumps can be deleterious to glia. In this review, we summarize recent advances in the understanding Ca2+ homeostasis in glial cells, the consequences of its alteration in cell demise as well as in neurological and psychiatric disorders that experience glial cell loss.

Broad-spectrum effects of 4-aminopyridine to modulate amyloid beta1-42-induced cell signaling and functional responses in human microglia

We investigated the modulating actions of the nonselective K(+) channel blocker 4-aminopyridine (4-AP) on amyloid beta (Abeta(1-42))-induced human microglial signaling pathways and functional processes. Whole-cell patch-clamp studies showed acute application of Abeta(1-42) (5 mum) to human microglia led to rapid expression of a 4-AP-sensitive, non-inactivating outwardly rectifying K(+) current (I(K)). Intracellular application of the nonhydrolyzable analog of GTP, GTPgammaS, induced an outward K(+) current with similar properties to the Abeta(1-42)-induced I(K) including sensitivity to 4-AP (IC(50) = 5 mm). Reverse transcriptase-PCR showed a rapid expression of a delayed rectifier Kv3.1 channel in Abeta(1-42)-treated microglia. Abeta(1-42) peptide also caused a slow, progressive increase in levels of [Ca(2+)](i) (intracellular calcium) that was partially blocked by 4-AP. Chronic exposure of human microglia to Abeta(1-42) led to enhanced p38 mitogen-activated protein kinase and nuclear factor kappaB expression with factors inhibited by 4-AP. Abeta(1-42) also induced the expression and production of the pro-inflammatory cytokines interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha, the chemokine IL-8, and the enzyme cyclooxygenase-2; 4-AP was effective in reducing all of these pro-inflammatory mediators. Additionally, toxicity of supernatant from Abeta(1-42)-treated microglia on cultured rat hippocampal neurons was reduced if 4-AP was included with peptide. In vivo, injection of Abeta(1-42) into rat hippocampus induced neuronal damage and increased microglial activation. Daily administration of 1 mg/kg 4-AP was found to suppress microglial activation and exhibited neuroprotection. The overall results suggest that 4-AP modulation of an Abeta(1-42)-induced I(K) (candidate channel Kv3.1) and intracellular signaling pathways in human microglia could serve as a therapeutic strategy for neuroprotection in Alzheimer's disease pathology.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Demyelination: the role of reactive oxygen and nitrogen species

This review summarises the role that reactive oxygen and nitrogen species play in demyelination, such as that occurring in the inflammatory demyelinating disorders multiple sclerosis and Guillain-Barré syndrome. The concentrations of reactive oxygen and nitrogen species (e.g. superoxide, nitric oxide and peroxynitrite) can increase dramatically under conditions such as inflammation, and this can overwhelm the inherent antioxidant defences within lesions. Such oxidative and/or nitrative stress can damage the lipids, proteins and nucleic acids of cells and mitochondria, potentially causing cell death. Oligodendrocytes are more sensitive to oxidative and nitrative stress in vitro than are astrocytes and microglia, seemingly due to a diminished capacity for antioxidant defence, and the presence of raised risk factors, including a high iron content. Oxidative and nitrative stress might therefore result in vivo in selective oligodendrocyte death, and thereby demyelination. The reactive species may also damage the myelin sheath, promoting its attack by macrophages. Damage can occur directly by lipid peroxidation, and indirectly by the activation of proteases and phospholipase A2. Evidence for the existence of oxidative and nitrative stress within inflammatory demyelinating lesions includes the presence of both lipid and protein peroxides, and nitrotyrosine (a marker for peroxynitrite formation). The neurological deficit resulting from experimental autoimmune demyelinating disease has generally been reduced by trial therapies intended to diminish the concentration of reactive oxygen species. However, therapies aimed at diminishing reactive nitrogen species have had a more variable outcome, sometimes exacerbating disease.

Microglia Kv1.3 channels contribute to their ability to kill neurons

Many CNS disorders involve an inflammatory response that is orchestrated by cells of the innate immune system: macrophages, neutrophils, and microglia (the endogenous CNS immune cell). Hence, there is considerable interest in anti-inflammatory strategies that target these cells. Microglia express Kv1.3 (KCNA3) channels, which we showed previously are important for their proliferation and the NADPH-mediated respiratory burst. Here, we demonstrate the potential for targeting Kv1.3 channels to control CNS inflammation. Rat microglia express Kv1.2, Kv1.3, and Kv1.5 transcripts and protein, but only a Kv1.3 current was detected. When microglia were activated with lipopolysaccharide or a phorbol ester, only the Kv1.3 transcript (but not protein) expression changed. Using a Transwell cell-culture system that allows separate drug treatment of microglia or neurons, we found that activated microglia killed postnatal hippocampal neurons through a process that requires Kv1.3 channel activity in microglia but not in neurons. A major neurotoxic molecule in this model was peroxynitrite, which is formed from superoxide and nitric oxide; thus, it is significant that Kv1.3 channel blockers reduced the respiratory burst, but not nitric oxide production, by the activated microglia. In addressing the biochemical pathway affected by Kv1.3 channel activity, we found that Kv1.3 acts via a different cellular mechanism from the broad-spectrum drug minocycline, which is often used in animal models of neuroinflammation. That is, the dose-dependent reduction in neuron killing by minocycline corresponded with a reduction in p38 mitogen-activated protein kinase activation in microglia; however, none of the Kv1.3 blockers affected p38 activation.

Regulation of microglial behavior by ion channel activity

Microglia play an important role in the central nervous system, where these cells, it is believed, have both neuroprotective and neurotoxic effects. In response to acute brain injury or during neurodegenerative and neuroinflammatory diseases, activated microglial cells undergo shape changes, migrate to the affected sites of neuronal damage, proliferate, and release a variety of substances, such as cytokines and reactive oxygen species (ROS). This review summarizes the physiological mechanisms underlying microglial activation and deactivation processes, with particular focus on the involvement of microglial ion channels. Microglial ion channels have been shown to be capable, by regulating membrane potential, cell volume, and intracellular ion concentrations, of modulating or facilitating proliferation, migration, cytokine secretion, shape changes, and the respiratory burst of microglial cells.

Integration of K+ and Cl- currents regulate steady-state and dynamic membrane potentials in cultured rat microglia

The role of ion channels and membrane potential (V(m)) in non-excitable cells has recently come under increased scrutiny. Microglia, the brain's resident immune cells, express voltage-gated Kv1.3 channels, a Kir2.1-like inward rectifier, a swelling-activated Cl(-) current and several other channels. We previously showed that Kv1.3 and Cl(-) currents are needed for microglial cell proliferation and that Kv1.3 is important for the respiratory burst. Although their mechanisms of action are unknown, one general role for these channels is to maintain a negative V(m). An impediment to measuring V(m) in non-excitable cells is that many have a very high electrical resistance, which makes them extremely susceptible to leak-induced depolarization. Using non-invasive V(m)-sensitive dyes, we show for the first time that the membrane resistance of microglial cells is several gigaohms; much higher than the seal resistance during patch-clamp recordings. Surprisingly, we observed that small current injections can evoke large V(m) oscillations in some microglial cells, and that injection of sinusoidal currents of varying frequency exposes a strong intrinsic electrical resonance in the 5- to 20-Hz frequency range in all microglial cells tested. Using a dynamic current clamp that we developed to actively compensate for the damage done by the patch-clamp electrode, we found that the V(m) oscillations and resonance were more prevalent and larger. Both types of electrical behaviour required Kv1.3 channels, as they were eliminated by the Kv1.3 blocker, agitoxin-2. To further determine how the ion currents integrate in these cells, voltage-clamp recordings from microglial cells displaying these behaviours were used to analyse the biophysical properties of the Kv1.3, Kir and Cl(-) currents. A mathematical model that incorporated only these three currents reproduced the observed V(m) oscillations and electrical resonance. Thus, the electrical behaviour of this 'non-excitable' cell type is much more complex than previously suspected, and might reflect a more common oversight in high resistance cells.

Inwardly rectifying K+ channels influence Ca2+ entry due to nucleotide receptor activation in microglia

The expression in microglia of two K+ channel populations, inwardly- and delayed outwardly rectifying channels (Kir, Kdr), is under the control of a variety of signals among which inflammatory and immunomodulatory agents. This makes K+ channels good candidates for the control of cell activities and for their adaptation to the changes of the functional state of the cell. Here we investigated on the role played by Kir channels in the control of cytoplasmic Ca2+ movements. In particular, we focused on those linked to nucleotide receptors, which are known to regulate a variety of functions in microglia. By a Fura-2-based video-imaging approach we recorded Ca2+ transients induced by P2 activation. These were composed of an initial peak, mainly due to release from endoplasmic reticulum, and of a long lasting plateau linked to Ca2+ influx through cation non-selective and capacitative channels. In patch-clamp experiments, we observed that Ba2+ (1-100 microM) could inhibit Kir current, but was not effective on Kdr and ATP-induced K+ current. By using Ba2+ as a specific blocker of Kir channels, we found that their inhibition caused a decrease of the Ca2+ level, especially at the end of the 20s long agonist application period. The effect of Ba2+ was mimicked by high K(+)-induced depolarization. We conclude that Kir channels contribute to modulate the amplitude and time course of the ATP-induced Ca2+ transient through the control of membrane potential. We suggest that microglial cells adapt signal transduction mechanisms to the changes of their functional state also by varying the expression and modulating the activity of inwardly rectifying K+ channels.

Regulation of a TRPM7-like current in rat brain microglia

Non-excitable cells use Ca2+ influx for essential functions but usually lack voltage-gated Ca2+ channels. The main routes of Ca2+ entry appear to be store-operated channels or Ca2+-permeable non-selective cation channels, of which the magnesium-inhibited cation (or magnesium-nucleotide-regulated metal cation) current has received considerable recent attention. This current appears to be produced by one of the recently cloned transient receptor potential (TRP) channels, TRPM7. In this study of rat microglia, we identified TRPM7 transcripts and a prevalent current with the hallmark biophysical and pharmacological features of TRPM7. This is the first identification of a TRPM7-like current in the brain. There is little known about how members of the TRPM sub-family normally become activated. Using whole-cell patch clamp recordings from rat microglia, we found that the TRPM7-like current activates spontaneously after break-in and that the current and its activation are inhibited by elevated intracellular Mg2+ but not affected by cell swelling or a wide range of intracellular Ca2+ concentrations. The TRPM7-like current in microglia appears to depend on tyrosine phosphorylation. It was inhibited by several tyrosine kinase inhibitors, including a peptide (Src 40-58) that was shown previously to inhibit Src actions, but not by inactive drugs or peptide analogues. The current did not depend on the cell activation state; i.e. it was the same in microglia recently removed from the brain or when cultured under a wide range of conditions that favor the resting or activated state. Because TRPM7 channels are permeable to Ca2+, this current may be important for microglia functions that depend on elevations in intracellular Ca2+.

Differential activation of subtype purinergic receptors modulates Ca(2+) mobilization and COX-2 in human microglia

We have studied modulation of purinergic receptors (P(2Y) and P(2X) subtypes) on changes in intracellular Ca(2+) [Ca(2+)](i) and expression and production of COX-2 in human microglia. Measurements using Ca(2+)-sensitive spectrofluorometry showed adenosine triphosphate (ATP) to cause rapid transient increases in [Ca(2+)](i). Application of ATP plus the P(2X) antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), or treatment with adenosine diphosphate-beta-S (ADP-beta-S), a selective P(2Y) agonist, led to a considerable prolongation in [Ca(2+)](i) responses compared with ATP. The prolonged time courses were consistent with sustained activation of store-operated channels (SOC) since SKF96365, an inhibitor of SOC, blocked this component of the response. RT-PCR data showed that microglia expressed no COX-2 either constitutively or following treatment of cells with ATP (100 microM for 8 h). However, treatment using ATP plus PPADS or with ADP-beta-S led to marked expression of COX-2. The enhanced COX-2 with ATP plus PPADS treatment was absent in the presence of SKF96365 or using Ca(2+)-free solution. Immunocytochemistry, using a specific anti-COX-2 antibody, also revealed a pattern of purinergic modulation whereby lack of P(2X) activation enhanced the production of COX-2 protein. These results suggest that modulation of subtypes of purinergic receptors regulates COX-2 in human microglia with a link involving SOC-mediated influx of Ca(2+).

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Calcium signaling in microglial cells

Receptor-mediated Ca(2+) signals are a common signal transduction mechanism in all living cells, including microglia. Recent years have brought major advances in our understanding of microglial Ca(2+) signaling. More than 20 receptor/ligand interactions leading to Ca(2+) signals in microglia have been described so far, and it seems that this is just the beginning. The literature has grown rapidly during the past few years, especially in regard to chemokine and ATP/UTP receptor signaling. This article presents a brief overview of the basics of Ca(2+) signaling, reviews the current literature on microglial Ca(2+) signaling, and discusses the current challenges and possible future directions of this emerging field.

Ion channels in cultured microglia

Inward and, depending on activation state, outward potassium currents are the dominant ion channels in microglial cells in culture. During transition between resting and activated phases, there is also an upregulated expression of stretch/swelling-activated chloride currents. Pharmacological blockade of the specific potassium channels does not prevent the transition, whereas blockade of chloride channels does, suggesting that this current may be involved in phase changes. Interestingly, this chloride current is far less studied than the potassium currents with regard to the different microglial phases. One puzzling finding when studying microglial state is that despite changes in current densities and membrane oscillations during transition, there is no evidence of an accompanying change in membrane potential. In other cells of the immune system, membrane oscillations and alterations in membrane potential are correlated with transitions in cellular phases. This discrepancy in microglia may be a result of the fact that almost all ion channel and membrane potential studies in culture are undertaken with concomitant dialysis of cytoplasm with pipette solution. Further complicating matters is that the few studies that use microglia in situ, find fundamental differences in ion channel current patterns of "resting" microglia as well as different temporal changes to pathological events or stimuli.

K+ channels and the microglial respiratory burst

Microglial activation following central nervous system damage or disease often culminates in a respiratory burst that is necessary for antimicrobial function, but, paradoxically, can damage bystander cells. We show that several K+ channels are expressed and play a role in the respiratory burst of cultured rat microglia. Three pharmacologically separable K+ currents had properties of Kv1.3 and the Ca2+/calmodulin-gated channels, SK2, SK3, and SK4. mRNA was detected for Kv1.3, Kv1.5, SK2, and/or SK3, and SK4. Protein was detected for Kv1.3, Kv1.5, and SK3 (selective SK2 and SK4 antibodies not available). No Kv1.5-like current was detected, and confocal immunofluorescence showed the protein to be subcellular, in contrast to the robust membrane localization of Kv1.3. To determine whether any of these channels play a role in microglial activation, a respiratory burst was stimulated with phorbol 12-myristate 13-acetate and measured using a single cell, fluorescence-based dihydrorhodamine 123 assay. The respiratory burst was markedly inhibited by blockers of SK2 (apamin) and SK4 channels (clotrimazole and charybdotoxin), and to a lesser extent, by the potent Kv1.3 blocker agitoxin-2.

Noninvasive measurement of hydrogen and potassium ion flux from single cells and epithelial structures

This review introduces new developments in a technique for measuring the movement of ions across the plasma membrane. With the use of a self-referencing ion-selective (Seris) probe, transport mechanisms can be studied on a variety of preparations ranging from tissues to single cells. In this paper we illustrate this versatility with examples from the vas deferens and inner ear epithelium to large and small single cells represented by mouse single-cell embryos and rat microglia. Potassium and hydrogen ion fluxes are studied and pharmacological manipulation of the signals are reported. The strengths of the self-referencing technique are reviewed with regard to biological applications, and the expansion of self-referencing probes to include electrochemical and enzyme-based sensors is discussed.

Beta-chemokines and human immunodeficiency virus type-1 proteins evoke intracellular calcium increases in human microglia

Activation of beta-chemokine receptors, co-receptors for human immunodeficiency virus type-1 (HIV-1), stimulates movement and secretion in microglia, possibly through a Ca(2+)-dependent mechanism. We studied chemokine activation of Ca(2+) signaling processes in microglia. Human fetal microglia were grown in primary culture and chemokine-induced increases in intracellular calcium concentration ([Ca(2+)](i)) were measured in single cells using indo-1-based microfluorimetry. Application of 50 ng/ml regulated on activation, normal T expressed and secreted (RANTES; 120 s) evoked responses in 26% of the microglia (187/719 cells). [Ca(2+)](i) increased from a basal level of 66+/-6 nM to peak at 268+/-23 nM (n=187). Chemokine-evoked responses rapidly desensitized as indicated by the rapid return to basal [Ca(2+)](i) levels in the maintained presence of RANTES. The removal of extracellular Ca(2+) or stimulation in the presence of Ni(2+) (2mM) or La(3+) (100 microM) blocked the RANTES-elicited [Ca(2+)](i) increase. The L-type calcium channel antagonist nimodipine (10 microM) inhibited the RANTES-mediated increase in [Ca(2+)](i) by 80+/-16%. Thus, the RANTES-evoked calcium transient appears to result from Ca(2+) influx with little if any release from intracellular stores. Application of gp120(clade) (E) and gp120(CM235) (50 ng/ml) neither mimicked nor antagonized the RANTES-evoked response. Application of 50 ng/ml eotaxin (120 s) evoked an increase in [Ca(2+)](i) in 13% of the human microglia in culture (61/469 cells). The HIV-1 regulatory protein Tat (50 ng/ml) increased the [Ca(2+)](i) in a subset of eotaxin-responsive cells (16/30). The L-type calcium channel antagonist nimodipine (3 microM) inhibited eotaxin- and Tat-mediated increases in [Ca(2+)](i) by 88+/-6% and 93+/-6%, respectively. Thus, activation of CCR3 appears to evoke Ca(2+) influx through L-type Ca(2+) channels.These results indicate that beta-chemokines, RANTES and eotaxin, activate a nimodipine sensitive Ca(2+) influx pathway in human fetal microglia. HIV-1 Tat protein mimicked chemokine-mediated Ca(2+) signaling and may modulate the migratory and secretory responses of microglia.

High concentrations of extracellular potassium enhance bacterial endotoxin lipopolysaccharide-induced neurotoxicity in glia-neuron mixed cultures

A sudden increase in extracellular potassium ions (K(+)) often occurs in cerebral ischemia and after brain trauma. This increase of extracellular K(+) constitutes the basis for spreading depression across the cerebral cortex, resulting in the expansion of neuronal death after ischemic and traumatic brain injuries. Besides spreading depression, it has become clear that cerebral inflammation also is a key factor contributing to secondary brain injury in acute neurological disorders. Experiments to validate the relationship between elevated levels of extracellular K(+) and inflammation have not been studied. This study aims to elucidate the roles of high concentrations of extracellular K(+) in bacterial endotoxin lipopolysaccharide-induced production of inflammatory factors. Increased concentration of KCl in the medium (20mM) significantly enhanced neurotoxicity by lipopolysaccharide in glia-neuron mixed cultures. To delineate the underlying mechanisms of increased neurotoxicity, the effects of high extracellular K(+) were examined by using mixed glial cultures. KCl at 20mM significantly enhanced nitrite, an index for nitric oxide, production by about twofold, and was pronounced from 24 to 48h, depending on the concentration of KCl. Besides nitric oxide production of tumor necrosis factor-alpha was also enhanced. The augmentative effects of high KCl on the production of inflammatory factors were probably due to the further activation of microglia, since high KCl also enhanced the production of tumor necrosis factor-alpha in microglia-enriched cultures. The increased production of nitrite by high K(+) was eliminated through use of a K(+)-blocker. Taken together, the results show that increases of extracellular K(+) concentrations in spreading depression augment lipopolysaccharide-elicited neurotoxicity, because production of inflammatory factors such as nitric oxide and tumor necrosis factor-alpha are potentiated. Since spreading depression and cerebral inflammation are important in acute neurological disorders, the present results suggest a biochemical mechanism: elevated extracellular K(+) concentrations augment glial inflammatory responses, and thus the neurotoxicity.

Suppression of the rat microglia Kv1.3 current by src-family tyrosine kinases and oxygen/glucose deprivation

Microglia activate following numerous acute insults to the brain, including oxygen/glucose deprivation (OGD), and both protein tyrosine kinases (PTKs) and K+ channels have been implicated in their activation. We identified Kv1.3 (voltage-gated potassium channel) protein in cultured rat microglia and confirmed that the native current is biophysically and pharmacologically similar to Kv1. 3. To explore whether src-family PTKs regulate the microglial Kv current, we first heterologously expressed Kv1.3 in a microglia-like cell line derived from neonatal rat brain (MLS-9). The resulting large Kv1.3 current was eliminated by co-transfecting the constitutively active PTK, v-src, then rapidly restored by the PTK inhibitor, lavendustin A. Acute activation of endogenous src kinases by a peptide activator significantly reduced the current, an effect that was mimicked by OGD. Similarly, in primary cultures of rat microglia, the endogenous Kv1.3-like current was inhibited by activating endogenous src-family PTKs and by OGD. Biochemical analysis showed that OGD increased the tyrosine phosphorylation of native Kv1.3 protein, which was alleviated by PTK inhibitors or reactive oxygen species (ROS) scavengers. Conversely, the basal level of Kv1.3 phosphorylation was decreased by PTK inhibitors or scavengers of ROS. Together, our results point to a post-insertional downregulation of the microglial Kv1.3-like current by oxidative stress and tyrosine phosphorylation. This interaction may be facilitated by a multiprotein complex because, in cultured microglia, the endogenous Kv1.3 and src proteins both bind to the scaffolding protein, post-synaptic density protein 95 (PSD-95). By associating with, and phosphorylating Kv1.3, src is well positioned to regulate microglial responses to oxidative stress.

Extracellular K(+) and opening of voltage-gated potassium channels activate T cell integrin function: physical and functional association between Kv1.3 channels and beta1 integrins

Elevated extracellular K(+) ([K(+)](o)), in the absence of "classical" immunological stimulatory signals, was found to itself be a sufficient stimulus to activate T cell beta1 integrin moieties, and to induce integrin-mediated adhesion and migration. Gating of T cell voltage-gated K(+) channels (Kv1.3) appears to be the crucial "decision-making" step, through which various physiological factors, including elevated [K(+)](o) levels, affect the T cell beta1 integrin function: opening of the channel leads to function, whereas its blockage prevents it. In support of this notion, we found that the proadhesive effects of the chemokine macrophage-inflammatory protein 1beta, the neuropeptide calcitonin gene-related peptide (CGRP), as well as elevated [K(+)](o) levels, are blocked by specific Kv1.3 channel blockers, and that the unique physiological ability of substance P to inhibit T cell adhesion correlates with Kv1.3 inhibition. Interestingly, the Kv1.3 channels and the beta1 integrins coimmunoprecipitate, suggesting that their physical association underlies their functional cooperation on the T cell surface. This study shows that T cells can be activated and driven to integrin function by a pathway that does not involve any of its specific receptors (i.e., by elevated [K(+)](o)). In addition, our results suggest that undesired T cell integrin function in a series of pathological conditions can be arrested by molecules that block the Kv1.3 channels.

Platelet-activating factor induced Ca(2+) signaling in human microglia

Increases in intracellular Ca(2+) concentration in human microglial cells in response to platelet-activating factor (PAF) were studied using Ca(2+)-sensitive fluorescence microscopy. In normal physiological solution (PSS), PAF-induced transient increases in [Ca2+](i) which recovered to baseline values within 200 s. Application of PAF in zero-Ca(2+) solution caused the peak response to be decreased to a value near 20% of that recorded in PSS suggesting a primary contribution of Ca(2+) influx for the [Ca2+](i) increase in PSS. To investigate PAF-induced Ca(2+) influx, the contents of intracellular stores were modulated using the SERCA blocker cyclopiazonic acid (CPA). The Ca(2+) signal induced by CPA (10 microM) in zero-Ca(2+) solution showed a peak response about 20% of the amplitude in the presence of external Ca(2+), suggesting the latter response included significant contributions from store-operated Ca(2+) entry. The influx of divalent cations with PAF or CPA was directly measured using Mn(2+) quenching of the fluorescence signal. Although both PAF and CPA induced a similar degree of Mn(2+) influx over time, the PAF effect was very rapid, whereas the CPA action was delayed and only evident about 200 s after application. Overall, the results show that the primary source of the PAF-induced increase of [Ca2+](i) in human microglia was the influx of Ca(2+) from the extracellular space and intracellular Ca(2+)-release contributed only a small part of the total Ca(2+) signal. Nevertheless, Ca(2+)-release induced by PAF (or CPA) serves as an important factor in controlling Ca(2+) entry presumably mediated by activation of store-operated-Ca(2+) channels.

Long-term activation of capacitative Ca2+ entry in mouse microglial cells

The cytoplasmic free calcium concentration ([Ca2+]i) was measured in cultured microglial cells with the Ca2+-sensitive fluorescent dye Fura-2 using a digital imaging system. Stimulation of P2 purinergic receptors by ATP or UTP always evoked a [Ca2+]i elevation. The ATP-induced Ca2+ response involved both Ca2+ influx through ionotropic receptors and Ca2+ release from intracellular pools, whereas UTP selectively stimulated intracellular Ca2+ release. When intracellular Ca2+ release was stimulated in the absence of extracellular Ca2+, the readmission of extracellular Ca2+ caused a large rebound [Ca2+]i increase. Following this rebound, [Ca2+]i did not return to the initial resting level, but remained for long periods of time (up to 20 min), at a new, higher steady-state level. Both the amplitude of the rebound Ca2+ transient and the new plateau level strongly correlated with the degree of intracellular Ca2+ depletion, indicating the activation of a store-operated Ca2+ entry pathway. The elevated steady-state [Ca2+]i level was associated with a significant increase in the plasma membrane permeability to Ca2+, as changes in extracellular Ca2+ were reflected in almost immediate changes of [Ca2+]i. Similarly, blocking plasma-lemmal Ca2+ channels with the non-specific agonist La3+ (50 microM) caused a decrease in [Ca2+]i, despite the continuous presence of Ca2+ ions in the extracellular medium. After the establishment of the new, elevated steady-state [Ca2+]i level, stimulation of P2U metabotropic purinoreceptors did not induce a [Ca2+]i response. In addition, application of either thapsigargin (1 microM) or carbonyl cyanide chlorophenyl hydrazone (10 microM) failed to affect [Ca2+]i. We conclude that the maximal depletion of intracellular Ca2+ stores in mouse brain microglia determines the long-term activation of a plasma membrane Ca2+ entry pathway. This activation appears to be associated with a significant decrease in the capability of the intracellular Ca2+ stores to take up cytosolic Ca2+ once they have been maximally depleted.

Ion channels in microglia (brain macrophages)

Microglia are immunocompetent cells in the brain that have many similarities with macrophages of peripheral tissues. In normal adult brain, microglial cells are in a resting state, but they become activated during inflammation of the central nervous system, after neuronal injury, and in several neurological diseases. Patch-clamp studies of microglial cells in cell culture and in tissue slices demonstrate that microglia express a wide variety of ion channels. Six different types of K+ channels have been identified in microglia, namely, inward rectifier, delayed rectifier, HERG-like, G protein-activated, as well as voltage-dependent and voltage-independent Ca2+-activated K+ channels. Moreover, microglia express H+ channels, Na+ channels, voltage-gated Ca2+ channels, Ca2+-release activated Ca2+ channels, and voltage-dependent and voltage-independent Cl- channels. With respect to their kinetic and pharmacological properties, most microglial ion channels closely resemble ion channels characterized in other macrophage preparations. Expression patterns of ion channels in microglia depend on the functional state of the cells. Microglial ion channels can be modulated by exposure to lipopolysaccharide or various cytokines, by activation of protein kinase C or G proteins, by factors released from astrocytes, by changes in the concentration of internal free Ca2+, and by variations of the internal or external pH. There is evidence suggesting that ion channels in microglia are involved in maintaining the membrane potential and are also involved in proliferation, ramification, and the respiratory burst. Further possible functional roles of microglial ion channels are discussed.

HERG-like K+ channels in microglia

A voltage-gated K+ conductance resembling that of the human ether-à-go-go-related gene product (HERG) was studied using whole-cell voltage-clamp recording, and found to be the predominant conductance at hyperpolarized potentials in a cell line (MLS-9) derived from primary cultures of rat microglia. Its behavior differed markedly from the classical inward rectifier K+ currents described previously in microglia, but closely resembled HERG currents in cardiac muscle and neuronal tissue. The HERG-like channels opened rapidly on hyperpolarization from 0 mV, and then decayed slowly into an absorbing closed state. The peak K+ conductance-voltage relation was half maximal at -59 mV with a slope factor of 18.6 mV. Availability, assessed by a hyperpolarizing test pulse from different holding potentials, was more steeply voltage dependent, and the midpoint was more positive (-14 vs. -39 mV) when determined by making the holding potential progressively more positive than more negative. The origin of this hysteresis is explored in a companion paper (Pennefather, P.S., W. Zhou, and T.E. DeCoursey. 1998. J. Gen. Physiol. 111:795-805). The pharmacological profile of the current differed from classical inward rectifier but closely resembled HERG. Block by Cs+ or Ba2+ occurred only at millimolar concentrations, La3+ blocked with Ki = approximately 40 microM, and the HERG-selective blocker, E-4031, blocked with Ki = 37 nM. Implications of the presence of HERG-like K+ channels for the ontogeny of microglia are discussed.

Reoxygenation increases the release of reactive oxygen intermediates in murine microglia

Respiratory burst activity of murine microglial cells was investigated in vitro under normoxic and hypoxic conditions with a chemoluminometric assay. Hypoxia for 24 hours reduced the release of extracellular reactive oxygen intermediates (ROIs), whereas reoxygenation increased the chemoluminescence more than sevenfold. Blockade of potassium channels inhibited the increase of oxidative burst after reoxygenation, indicating that potassium ions, which were increased in the supernatant of hypoxic microglial cells, were involved in this activation process. Also, blockade of voltage-gated calcium channels with nifedipine attenuated the increased release of ROIs. With fura-2 analysis, it was shown that the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase by potassium ions was mediated by calcium influx via voltage-gated calcium channels. Thus, influx of calcium ions through voltage-gated channels activates the NADPH oxidase in microglial cells during reoxygenation. By the increased production of ROIs, microglial cells may add to the reperfusion injury after ischemia in vivo.