Ion channels of human microglia in culture

Retinal Organoid Microenvironment Enhanced Bioactivities of Microglia-Like Cells Derived From HiPSCs

Purpose

Microglia-like cells derived from stem cells (iMG) provide a plentiful cell source for studying the functions of microglia in both normal and pathological conditions. Our goal is to establish a simplified and effective method for generating iMG in a precisely defined system. Additionally, we aim to achieve functional maturation of iMG through coculture with retinal organoids.

Methods

In this study, iMG were produced under precisely defined conditions. They were subjected to LPS and poly IC stimulation. Additionally, we examined distinct phenotypic and functional variances between iMG and HMC3, a commonly used human microglia cell line. To investigate how the retinal cell interaction enhances microglial properties, iMG were cocultured with retinal organoids, producing CC-iMG. We performed RNA sequencing, electrophysiological analysis, and transmission electron microscope (TEM) to examine the maturation of CC-iMG compared to iMG.

Results

Our results demonstrated that iMG performed immune-responsive profiles closely resembling those of primary human microglia. Compared to HMC3, iMG expressed a higher level of typical microglial markers and exhibited enhanced phagocytic activity. The transcriptomic analysis uncovered notable alterations in the ion channel profile of CC-iMG compared to iMG. Electrophysiological examination demonstrated a heightened intensity of inward- and outward-rectifying K+ currents in CC-iMG. Furthermore, CC-iMG displayed elevated numbers of lysosomes and mitochondria, coupled with increased phagocytic activity.

Conclusions

These findings contribute to advancing our understanding of human microglial biology, specifically in characterizing and elucidating the functions of CC-iMG, thereby offering an in vitro microglial model for future scientific research and potential clinical applications in cell therapy.

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Different Age Related Neurological and Cardiac Effects of Verapamil on a Transgenic Mouse Model of Alzheimer's Disease

Dementias are the third cause of the disability-adjusted life-years (DALYs) worldwide with Alzheimer’s (AD) having the highest prevalence. Despite ample research in the field, therapeutic options are limited. However, with the increase in lifespan, a larger number of AD patients will receive other medication for the evermore-increased number of comorbidities that such patients face. The purpose of this study was to evaluate the neurological and cardiac effects of verapamil, on C57BL/6J-TgN (Thy1-APPKM670/671NL; Thy1-PS1L166P (APP) mice. The daily administration of 3.5mg/kg of verapamil for 28 days revealed different effects on young and aged APP mice. While young animals showed less anxiety and improved short-term memory with minimal cardiac effects (an increase in the duration of ventricular depolarization), aged ones did not present behavioral improvements, but with a decrease in the duration of ventricular depolarizing. Repolarization effects of verapamil were similar in young and aged animals, except for the duration of the ST segment that was longer in aged animals. Considering our results, the use of calcium blockers in AD patients should take into consideration the stage of the disease, as different effects could be seen at different stages of AD, in our model.

Large-conductance calcium-activated potassium channels mediate lipopolysaccharide-induced activation of murine microglia

Large-conductance calcium-activated potassium (BK) channels are ubiquitously expressed in most cell types where they regulate many cellular, organ, and organismal functions. Although BK currents have been recorded specifically in activated murine and human microglia, it is not yet clear whether and how the function of this channel is related to microglia activation. Here, using patch-clamping, Griess reaction, ELISA, immunocytochemistry, and immunoblotting approaches, we show that specific inhibition of the BK channel with paxilline (10 μm) or siRNA-mediated knockdown of its expression significantly suppresses lipopolysaccharide (LPS)-induced (100 ng/ml) BV-2 and primary mouse microglial cell activation. We found that membrane BK current is activated by LPS at a very early stage through Toll-like receptor 4 (TLR4), leading to nuclear translocation of NF-κB and to production of inflammatory cytokines. Furthermore, we noted that BK channels are also expressed intracellularly, and their nuclear expression significantly increases in late stages of LPS-mediated microglia activation, possibly contributing to production of nitric oxide, tumor necrosis factor-α, and interleukin-6. Of note, a specific TLR4 inhibitor suppressed BK channel expression, whereas an NF-κB inhibitor did not. Taken together, our findings indicate that BK channels participate in both the early and the late stages of LPS-stimulated murine microglia activation involving both membrane-associated and nuclear BK channels.

Morphological changes, nitric oxide production, and phagocytosis are triggered in vitro in microglia by bloodstream forms of Trypanosoma brucei

The flagellated parasite Trypanosoma brucei is the causative agent of Human African Trypanosomiasis (HAT). By a mechanism not well understood yet, trypanosomes enter the central nervous system (CNS), invade the brain parenchyma, and cause a fatal encephalopathy if is not treated. Trypanosomes are fast dividing organisms that, without any immune response, would kill the host in a short time. However, infected individuals survive either 6-12 months or more than 3 years for the acute and chronic forms, respectively. Thus, only when the brain defense collapses a lethal encephalopathy will occur. Here, we evaluated interactions between trypanosomes and microglial cells, which are the primary immune effector cells within the CNS. Using co-cultures of primary microglia and parasites, we found clear evidences of trypanosome phagocytosis by microglial cells. Microglia activation was also evident; analysis of its ultrastructure showed changes that have been reported in activated microglia undergoing oxidative stress caused by infections or degenerative diseases. Accordingly, an increase of the nitric oxide production was detected in supernatants of microglia/parasite co-cultures. Altogether, our results demonstrate that microglial cells respond to the presence of the parasite, leading to parasite's engulfment and elimination.

Microglia in Alzheimer's Disease: A Role for Ion Channels

Alzheimer's disease is the most common form of dementia, it is estimated to affect over 40 million people worldwide. Classically, the disease has been characterized by the neuropathological hallmarks of aggregated extracellular amyloid-β and intracellular paired helical filaments of hyperphosphorylated tau. A wealth of evidence indicates a pivotal role for the innate immune system, such as microglia, and inflammation in the pathology of Alzheimer's disease. The over production and aggregation of Alzheimer's associated proteins results in chronic inflammation and disrupts microglial clearance of these depositions. Despite being non-excitable, microglia express a diverse array of ion channels which shape their physiological functions. In support of this, there is a growing body of evidence pointing to the involvement of microglial ion channels contributing to neurodegenerative diseases such as Alzheimer's disease. In this review, we discuss the evidence for an array of microglia ion channels and their importance in modulating microglial homeostasis and how this process could be disrupted in Alzheimer's disease. One promising avenue for assessing the role that microglia play in the initiation and progression of Alzheimer's disease is through using induced pluripotent stem cell derived microglia. Here, we examine what is already understood in terms of the molecular underpinnings of inflammation in Alzheimer's disease, and the utility that inducible pluripotent stem cell derived microglia may have to advance this knowledge. We outline the variability that occurs between the use of animal and human models with regards to the importance of microglial ion channels in generating a relevant functional model of brain inflammation. Overcoming these hurdles will be pivotal in order to develop new drug targets and progress our understanding of the pathological mechanisms involved in Alzheimer's disease.

Potassium channel expression and function in microglia: Plasticity and possible species variations

Potassium channels play important roles in microglia functions and thus constitute potential targets for the treatment of neurodegenerative diseases like Alzheimer, Parkinson and stroke. However, uncertainty still prevails as to which potassium channels are expressed and at what levels in different species, how the expression pattern changes upon activation with M1 or M2 polarizing stimuli compared with more complex exposure paradigms, and - most importantly - how these findings relate to the in vivo situation. In this mini-review we discuss the functional potassium channel expression pattern in cultured neonatal mouse microglia in the light of data obtained previously from animal disease models and immunohistochemical studies and compare it with a recent study of adult human microglia isolated from epilepsy patients. Overall, microglial potassium channel expression is very plastic and possibly shows species differences and therefore should be studied carefully in each disease setting and respective animal models.

Mechanisms Underlying Interferon-γ-Induced Priming of Microglial Reactive Oxygen Species Production

Microglial priming and enhanced reactivity to secondary insults cause substantial neuronal damage and are hallmarks of brain aging, traumatic brain injury and neurodegenerative diseases. It is, thus, of particular interest to identify mechanisms involved in microglial priming. Here, we demonstrate that priming of microglia with interferon-γ (IFN γ) substantially enhanced production of reactive oxygen species (ROS) following stimulation of microglia with ATP. Priming of microglial ROS production was substantially reduced by inhibition of p38 MAPK activity with SB203580, by increases in intracellular glutathione levels with N-Acetyl-L-cysteine, by blockade of NADPH oxidase subunit NOX2 activity with gp91ds-tat or by inhibition of nitric oxide production with L-NAME. Together, our data indicate that priming of microglial ROS production involves reduction of intracellular glutathione levels, upregulation of NADPH oxidase subunit NOX2 and increases in nitric oxide production, and suggest that these simultaneously occurring processes result in enhanced production of neurotoxic peroxynitrite. Furthermore, IFNγ-induced priming of microglial ROS production was reduced upon blockade of Kir2.1 inward rectifier K⁺ channels with ML133. Inhibitory effects of ML133 on microglial priming were mediated via regulation of intracellular glutathione levels and nitric oxide production. These data suggest that microglial Kir2.1 channels may represent novel therapeutic targets to inhibit excessive ROS production by primed microglia in brain pathology.

Quantification of the functional expression of the Ca2+ -activated K+ channel KCa 3.1 on microglia from adult human neocortical tissue

The K_Ca 3.1 channel (KCNN4) is an important modulator of microglia responses in rodents, but no information exists on functional expression on microglia from human adults. We isolated and cultured microglia (max 1% astrocytes, no neurons or oligodendrocytes) from neocortex surgically removed from epilepsy patients and employed electrophysiological whole-cell measurements and selective pharmacological tools to elucidate functional expression of K_Ca 3.1. The channel expression was demonstrated as a significant increase in the voltage-independent current by NS309, a K_Ca 3.1/K_Ca 2 activator, followed by full inhibition upon co-application with NS6180, a highly selective K_Ca 3.1 inhibitor. A major fraction (79%) of unstimulated human microglia expressed K_Ca 3.1, and the difference in current between full activation and inhibition (ΔK_Ca 3.1) was estimated at 292 ± 48 pA at -40 mV (n = 75), which equals at least 585 channels per cell. Serial K_Ca 3.1 activation/inhibition significantly hyperpolarized/depolarized the membrane potential. The isolated human microglia were potently activated by lipopolysaccharide (LPS) shown as a prominent increase in TNF-α production. However, incubation with LPS neither changed the K_Ca 3.1 current nor the fraction of K_Ca 3.1 expressing cells. In contrast, the anti-inflammatory cytokine IL-4 slightly increased the K_Ca 3.1 current per cell, but as the membrane area also increased, there was no significant change in channel density. A large fraction of the microglia also expressed a voltage-dependent current sensitive to the K_Ca 1.1 modulators NS1619 and Paxilline and an inward-rectifying current with the characteristics of a K_ir channel. The high functional expression of K_Ca 3.1 in microglia from epilepsy patients accentuates the need for further investigations of its role in neuropathological processes. GLIA 2016;64:2065-2078.

Cytokines and Virus Proteins: Modulators of Glial Electrophysiological Properties

Cytokines are released during acute and chronic inflammatory diseases of the CNS and activate receptors on glial cells, thereby inducing various effects such as proliferation, expression of major histocompatibility complex genes or secretion of growth factors. Here, we summarize current evidence indicating that K+ currents, Ca 2+ currents, and the activity of ion transporters on astrocytes, microglial cells, and oligodendrocytes are also affected by cytokines. In disease states with associated elevated cytokine titers, such alterations in electrophysiological properties of glial cells might contribute to the patho genesis of neurological symptoms. NEUROSCIENTIST 5:142-146, 1999

Transient Receptor Potential Channels in Microglia: Roles in Physiology and Disease

Microglia modulate the nervous system cellular environment and induce neuroprotective and neurotoxic effects. Various molecules are involved in these processes, including families of ion channels expressed in microglial cells, such as transient receptor potential (TRP) channels. TRP channels comprise a family of non-selective cation channels that can be activated by mechanical, thermal, and chemical stimuli, and which contribute to the regulation of intracellular calcium concentrations. TRP channels have been shown to be involved in cellular processes such as osmotic regulation, cytokine production, proliferation, activation, cell death, and oxidative stress responses. Given the significance of these processes in microglial activity, studies of TRP channels in microglia have focused on determining their roles in both neuroprotective and neurotoxic processes. TRP channel activity has been proposed to play an important function in neurodegenerative diseases, ischemia, inflammatory responses, and neuropathic pain. Modulation of TRP channel activity may thus be considered as a potential therapeutic strategy for the treatment of various diseases associated with alterations of the central nervous system (CNS). In this review, we describe the expression of different subfamilies of TRP channels in microglia, focusing on their physiological and pathophysiological roles, and consider their potential use as therapeutic targets in CNS diseases.

Microglial K(+) channel expression in young adult and aged mice

The K(+) channel expression pattern of microglia strongly depends on the cells' microenvironment and has been recognized as a sensitive marker of the cells' functional state. While numerous studies have been performed on microglia in vitro, our knowledge about microglial K(+) channels and their regulation in vivo is limited. Here, we have investigated K(+) currents of microglia in striatum, neocortex and entorhinal cortex of young adult and aged mice. Although almost all microglial cells exhibited inward rectifier K(+) currents upon membrane hyperpolarization, their mean current density was significantly enhanced in aged mice compared with that determined in young adult mice. Some microglial cells additionally exhibited outward rectifier K(+) currents in response to depolarizing voltage pulses. In aged mice, microglial outward rectifier K(+) current density was significantly larger than in young adult mice due to the increased number of aged microglial cells expressing these channels. Aged dystrophic microglia exhibited outward rectifier K(+) currents more frequently than aged ramified microglia. The majority of microglial cells expressed functional BK-type, but not IK- or SK-type, Ca(2+) -activated K(+) channels, while no differences were found in their expression levels between microglia of young adult and aged mice. Neither microglial K(+) channel pattern nor K(+) channel expression levels differed markedly between the three brain regions investigated. It is concluded that age-related changes in microglial phenotype are accompanied by changes in the expression of microglial voltage-activated, but not Ca(2+) -activated, K(+) channels.

Voltage-gated proton channel HV1 in microglia

Microglia are brain resident immune cells and their functions are implicated in both the normal and diseased brain. Microglia express a plethora of ion channels, including K(+) channels, Na(+) channels, TRP channels, Cl(-) channels, and proton channels. These ion channels play critical roles in microglial proliferation, migration, and production/release of cytokines, chemokines, and neurotoxic or neurotrophic substances. Among microglial ion channels, the voltage-gated proton channel HV1 is a recently cloned ion channel that rapidly removes protons from depolarized cytoplasm and is highly expressed in the immune system. However, the function of microglial HV1 in the brain is poorly understood. Recent studies showed that HV1 is selectively expressed in microglia but not neurons in the brain. At the cellular level, microglial HV1 regulates intracellular pH and aids in NADPH oxidase-dependent generation of reactive oxygen species. In a mouse model of middle cerebral artery occlusion, microglial HV1 contributes to neuronal cell death and ischemic brain damage. This review discusses the discovery, properties, regulation, and pathophysiology of microglial HV1 proton channel in the brain.

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.

Microglia cell culture: a primer for the novice

Microglial cells are the resident immune cells of the central nervous system. Progress in the recent decade has clearly established that microglial cells participate or even actively drive neurological disease. Much of our current knowledge has been generated by investigating microglial cells in cell culture. The aim of this chapter is to give the uninitiated a basic and adaptable protocol for the culturing of microglial cells. We discuss the challenges of microglial cell culture and provide a collection of tips which reflect our 25+ years of collective experience.

Activation of microglia depends on Na+/H+ exchange-mediated H+ homeostasis

H(+) extrusion is important for sustained NADPH oxidase activation after "respiratory" burst in macrophage/microglia activation. In this study, we investigated the role of Na(+)/H(+) exchanger isoform 1 (NHE-1) in activation of microglia after lipopolysaccharide (LPS) or oxygen and glucose deprivation and reoxygenation (OGD/REOX) exposure. NHE-1 functioned in maintaining basal pH(i) of immortalized M4T.4 microglia or mouse primary microglia. Pharmacological inhibition of NHE-1 activity with the potent inhibitor cariporide [HOE 642 (4-isopropyl-3-methylsulfonyl-benzoyl-guanidine-methanesulfonate)] abolished pH(i) regulation in microglia under basal conditions. Activation of microglia either by LPS, phorbol myristate acetate, or OGD/REOX accelerated pH(i) regulation and caused pH(i) elevation, which was accompanied with an increase in [Na(+)](i) and [Ca(2+)](i) as well as production of superoxide anion and cytokines. Interestingly, inhibition of NHE-1 not only abolished pH(i) regulation but also reduced production of superoxide anion as well as expression of cytokines and inducible nitric oxide synthase. Together, these results reveal that there was a concurrent activation of NHE-1 in microglia in response to proinflammatory stimuli. The study suggests that NHE-1 functions to maintain microglial pH(i) homeostasis allowing for sustained NADPH oxidase function and "respiratory" burst.

Charge compensation for NADPH oxidase activity in microglia in rat brain slices does not involve a proton current

The membrane properties of isolated cultured microglia have been extensively studied but it is important to understand their properties in situ, where they protect the brain against infection, but also contribute to neurodegenerative diseases. Microglia and macrophages attack bacteria by generating reactive oxygen species, a process which involves NADPH oxidase pumping electrons out across the cell membrane. The resulting inward current evokes a depolarization, which would inhibit the activity of the NADPH oxidase if there were no charge-compensating current which moves positive charge out across the membrane. The mechanism of this charge compensation is controversial. In neutrophils and in cultured microglia a depolarization-activated H(+) conductance has been proposed to provide charge compensation, and also to remove protons generated intracellularly by the NADPH oxidase. Alternatively, a depolarization-activated K(+) conductance has been proposed to mediate charge compensation. Here we show that in microglia, either in the resting state or when activated by the bacterial coat component lipopolysaccharide, both in acute and in cultured hippocampal slices, no significant H(+) current is detectable. This implies that the membrane properties of microglia in their normal cellular environment differ from those of cultured microglia (similarly, microglia generated a current in response to ATP but, unlike in culture, not to glutamate or GABA). Furthermore, the K(+) current (Kv1.3) that is activated by lipopolysaccharide is inactivated by depolarization, making it unsuitable for mediating charge compensation on a long time scale at positive voltages. Instead, charge compensation may be mediated by a previously undescribed non-selective cation current.

Voltage-gated proton channels

The history of research on voltage-gated proton channels is recounted, from their proposed existence in dinoflagellates by Hastings in 1972 and their demonstration in snail neurons by Thomas and Meech in 1982 to the discovery in 2006 (after a decade of controversy) of genes that unequivocally code for proton channels. Voltage-gated proton channels are perfectly selective for protons, conduct deuterons half as well, and the conductance is strongly temperature dependent. These properties are consistent with a conduction mechanism involving hydrogen-bonded-chain transfer, in which the selectivity filter is a titratable amino acid residue. Channel opening is regulated stringently by pH such that only outward current is normally activated. Main functions of proton channels include acid extrusion from cells and charge compensation for the electrogenic activity of the phagocyte NADPH oxidase. Genetic approaches hold the promise of rapid progress in the near future.

Lipopolysaccharide-induced down-regulation of Ca2+ release-activated Ca2+ currents (I CRAC) but not Ca2+-activated TRPM4-like currents (I CAN) in cultured mouse microglial cells

Microglia are the main immunocompetent cells of the mammalian central nervous system (CNS). Activation of cultured microglial cells and subsequent release of nitric oxide and cytokines critically depends on intracellular calcium levels. Since microglia undergo dramatic morphological, biochemical and electrophysiological changes in response to pathological events in the CNS, we investigated temporal changes in expression levels of ion channels involved in cellular calcium homeostasis in mouse cortical microglial cells in culture. Specifically, we assessed the inward and delayed outward rectifier potassium currents (I IRK and I DRK), calcium (Ca2+) release-activated Ca2+ currents (I CRAC) and Ca2+-activated TRPM4-like currents (I CAN) in non-activated microglia and cells that were activated by exposure to lipopolysaccharide (LPS) between 3 and 48 h. Unstimulated microglial cells, subcultured from an astrocyte coculture, typically exhibited a ramified, rod-shaped morphology. During the first 3 days of culture cell size and shape were maintained, but the percentage of cells showing prominent I IRK went up and those expressing I DRK went down. Cells retaining I DRK exhibited smaller amplitudes, whereas those of I IRK and I CRAC were not affected. However, after 24 h of exposure to 1 microg ml(-1) LPS, most cells showed an amoeboid ('fried egg'-shaped) morphology with a 62% increase in cell capacitance. At that point in time, only 14% of the cells revealed I IRK and 3% had I DRK exclusively, whereas the majority of cells expressed both currents. The amplitudes of I CRAC and I IRK progressively decreased after stimulation, whereas I DRK transiently reached a maximum after 6 h of LPS exposure and then returned to pre-stimulation expression levels. Cultured microglia also revealed TRPM4-like, Ca2+-activated non-selective currents (I CAN) with an EC50 of 1.2 microm [Ca2+]i. The expression levels of this current did not change significantly during and after 24 h of LPS exposure. We propose that LPS-induced down-regulation of I IRK and I CRAC will reduce the cell's capacity to produce significant calcium influx upon receptor activation and result in decreased sensitivity to exogenous stimulation. In this scenario, I CAN expression would remain constant, although its activity would automatically be reduced due to the diminished calcium influx capacity of the cell.

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.

Topics on the Na+/Ca2+ Exchanger: Responses of Na+/Ca2+ Exchanger to Interferon-γ and Nitric Oxide in Cultured Microglia

The Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) levels, but little is known about the functional role of NCX in microglia. To clarify the role of NCX in microglia, we studied the responses of NCX to pathological conditions such as interferon-gamma or nitric oxide (NO) exposure. Treatment with interferon-gamma caused a biphasic increase in NCX activity. The delayed increase in NCX activity was accompanied by increases in the mRNA and protein levels. Pharmacological studies show that protein kinase C and tyrosine kinase are involved in the transient and delayed increases in NCX activity, and the extracellular signal-regulated protein kinase is involved in the delayed increase in NCX activity. On the other hand, NO causes apoptotic cell death in cultured microglia. We observed, using the specific NCX inhibitor SEA0400, that NO activates NCX activity and NCX is involved in NO-induced depletion of Ca(2+) in the endoplasmic reticulum (ER), leading to ER stress. These results suggest that NCX is involved in the regulation of Ca(2+) levels in the ER. The responses of NCX to interferon-gamma and NO implies that NCX plays a key role in microglial function.

Mitochondrial ligand inhibits store-operated calcium influx and COX-2 production in human microglia

We used calcium sensitive fluorescence microscopy to investigate the actions of PK11195, a ligand for the mitochondrial peripheral benzodiazepine receptor (PBR), to modulate Ca2+ influx through store-operated channels (SOC) in human microglia. PK11195 effectively blocked SOC-mediated Ca2+ influx induced by platelet-activating factor (PAF) in a dose-dependent manner (IC50 of 9 microM). A prolonged SOC-mediated Ca2+ entry was also induced using the sarcoplasmic endoreticulum Ca2+-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA) to deplete intracellular endoplasmic reticulum (ER) stores; a single concentration of PK11195 (at 20 microM) reduced SOC-mediated Ca2+ influx by 78%. RT-PCR and immunocytochemical analysis results showed PK11195 also inhibited the expression and production of cyclooxygenase-2 (COX-2) triggered by PAF stimulation. These results suggest that activation of the PBR in mitochondria is linked to reduced entry of Ca2+ through plasmalemmal SOC and subsequent modulation of cellular functions in human microglia.

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.

Purinergic mediated changes in Ca2+ mobilization and functional responses in microglia: effects of low levels of ATP

Microglia, the immune effector cells of the brain, are stimulated by a diversity of agents to transiently increase levels of intracellular calcium ([Ca2+]i). Changes in [Ca2+]i induced by compounds such as adenosine triphosphate (ATP) serve important roles in cellular signal transduction linking stimuli with cellular functional responses. Purinergic responses in microglia, like that in other cells, are mediated by two families of receptors classified as P2Y and P2X. Activation of metabotropic receptors (P2YR) leads to increased [Ca2+]i due to depletion of intracellular stores, a process that can trigger activation of Ca2+ entry through plasmalemmal store-operated channels (SOC). Activation of ionotropic receptors (P2XR) is associated with influx of Na+ and Ca2+ and efflux of K+ through nonselective cationic channels, leading to cellular depolarization. An intriguing property of purinergic stimulation of microglia is the dependence of cellular responses on agonist concentration. As one example, activation of the subtype P2X7R by higher levels of ATP (millimolar range), leads to a marked enhancement in microglial secretion of inflammatory mediators. Other members of the ionotropic P2XR family sensitive to lower levels of ATP, however, are also important in mediating microglial inflammatory responses in brain. At lower concentrations of ATP (100 microM), activation of SOC in human microglia is not only coupled to P2YR-dependent depletion of internal stores, but is also modulated by ATP binding to a P2XR (not P2X7R). The modulation is consistent with a P2XR-mediated influx of Na+ and inhibition of SOC by depolarization. In this review, a primary focus is placed on the effects of low concentrations of ATP (< or =100 microM) to induce changes in [Ca2+]i and modify functional processes in microglia. In essence, responses mediated by purinergic receptors other than P2X7R are considered.

Kinin receptors in cultured rat microglia

Kinins are produced and act at the site of injury and inflammation in various tissues. They are likely to initiate a particular cascade of inflammatory events, which evokes physiological and pathophysiological responses including an increase in blood flow and plasma leakage. In the central nervous system (CNS), kinins are potent stimulators of the production and release of pro-inflammatory mediators represented by prostanoids and cytotoxins. They are known to induce neural tissue damage. Many of the cytotoxins such as cytokines and free radicals and prostanoids are released from glial cells. Among glial cells, astrocytes and oligodendrocytes are known to possess bradykinin (BK) B(2) receptors that phosphoinositide (PI) turnover and raise intracellular Ca(2+) concentration. The presence of bradykinin receptors in microglia has been of great significance. We recently showed that rat primary microglia express kinin receptors. In resting microglia, B(2) receptors but not B(1) receptors are expressed. When the microglia are activated by bradykinin, B(1) receptors are up-regulated, while B(2) receptors are down-regulated. As observed in other glial cells, electrophysiological measurements suggest that B(2) receptors in phosphoinositide turnover and intracellular Ca(2+) concentration in microglia. Release of cytotoxins is likely consequent upon the activation of BK receptors. Our study provides the first evidence that microglia express functional kinin receptors and suggests that microglia play an important role in CNS inflammatory responses.

Chemokine modulation of high-conductance Ca2+-sensitive K+ currents in microglia from human hippocampi

During acute pathological processes, microglia transform into an activated state characterized by a defined morphology and current profile, and are recruited to injury sites by chemokines. No information is available on the ion channels and the mode of action of chemokines in microglia in brain slices from humans with a chronic pathology. Thus, patch-clamp recordings of microglia were performed in hippocampal slices from seven patients who underwent surgery for pharmaco-resistant epilepsy. Cells were identified as microglia by positive labelling with fluorescein-conjugated tomato lectin before recording. All the recorded cells had an ameboid morphology characteristic of activated microglia. However, they had a high input resistance (3.6 G omega), a zero-current resting potential of -16 mV, and lacked Na+ currents, inwardly rectifying and delayed rectifying K+ currents such as non-activated microglia. Importantly, recorded cells expressed Ca2+-sensitive outward currents that activated at 0 mV with non-buffered intracellular Ca2+ and were sensitive to 1 mm tetraethylammonium (TEA). The estimated single-channel conductances were 187 pS in cell-attached and 149 pS in outside-out patches, similar to those of high-conductance Ca2+-dependent K+ channels. The chemokine MIP1-alpha increased whole-cell outward current amplitudes measured at +60 mV by a factor of 3.3. Thus, microglia in hippocampi from epileptic patients express high-conductance Ca2+-dependent K+ channels that are modulated by the chemokine MIP1-alpha. This modulation may contribute to the migratory effect of MIP1-alpha on microglia.

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.

Generation and characterization of human hybrid neurons produced between embryonic CNS neurons and neuroblastoma cells

A human hybrid neuronal cell line A1 has been generated by somatic fusion between a human fetal cerebral neuron and a human neuroblastoma cell, and RT-PCR, immunochemical, and electrophysiological studies of the hybrid cells indicated that the cells express faithfully of morphological, immunochemical, physiological, and genetic features of human cerebral neurons. A1 hybrid neurons express neuron-specific markers such as neurofilament-L (NF-L), NF-M, NF-H, MAP-2, and beta tubulin III. A1 human hybrid neurons express messages for various cytokines and cytokine receptors which are similar to parental human CNS neurons and different from the other parental cell line, SK-SH-SY5Y neuroblastoma. A1 hybrid neurons also express messages for choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), and glutamic acid decarboxylase (GAD), indicating that they could differentiate into various subsets of neuronal types. Whole-cell patch clamp experiments showed that A1 hybrid neurons expressed Na+ currents, which were completely blocked by tetrodotoxin. In addition, depolarizing and hyperpolarizing voltage clamp steps evoked respective outward and inward K+ currents in these cells. When A1 hybrid neurons were exposed to beta amyloid for 72 hr, there was three-fold increase in TUNEL positive cells over controls, indicating that beta amyloid is neurotoxic to A1 hybrid neurons. The present study indicates that the A1 human hybrid neuronal cell line should serve as a valuable in vitro model for studies of biology, physiology, and pathology of human neurons in health and disease.

Chloride/anion channels in glial cell membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.

Interferon-gamma acutely induces calcium influx in human microglia

The acute actions of the cytokine, interferon-gamma (IFN-gamma), on intracellular calcium [Ca(2+)](i) levels in human microglia were investigated. In the presence of a calcium-containing physiological solution (Ca(2+)-PSS), IFN-gamma caused a progressive increase in [Ca(2+)](i) to a plateau level with a mean rate of increase of 0.81 +/- 0.17 nM/s and mean amplitude of 102 +/- 12 nM (n = 67 cells). Washout of the cytokine did not alter the plateau established with IFN-gamma in Ca(2+)-PSS; however, introduction of a Ca(2+)-free PSS diminished [Ca(2+)](i) to baseline levels. The decrease in [Ca(2+)](i) with Ca(2+)-free PSS would indicate that the response to IFN-gamma was mediated by an influx pathway. This result was confirmed in separate experiments showing the lack of an induced change in [Ca(2+)](i) with IFN-gamma applied in Ca(2+)-free PSS. The increase in [Ca(2+)](i) induced in Ca(2+)-PSS was reduced to near baseline levels when the external solution contained low Cl(-) in the maintained presence of IFN-gamma suggesting that cellular depolarization inhibited the cytokine mediated entry pathway. The compound SKF96365, which blocks store operated influx of Ca(2+) in human microglia, was ineffective in altering the increase in [Ca(2+)](i), however, La(3+) completely inhibited the Ca(2+) response induced by IFN-gamma. Whole-cell patch clamp studies showed no effect of IFN-gamma to alter outward currents and inward rectifier K(+) currents. The influx of Ca(2+) may serve a signaling role in microglia linking IFN-gamma to functional responses of the cells to infiltrating T lymphocytes into the central nervous system (CNS) during inflammatory processes.

Human neural stem cells: electrophysiological properties of voltage-gated ion channels

We have characterized the profile of membrane currents in an immortalized human neural stem cell line, HB1.F3 cells, using whole-cell patch clamp technique. Human neural stem cell line generated from primary cell cultures of embryonic human telencephalon using a replication-incompetent retroviral vector containing v-myc expresses nestin, a cell type-specific marker for neural stem cells. The human neural stem cells expressed both outward and inward K(+) currents with no evidence for Na(+) currents. The density of the outward, delayed rectifying type K(+) current was 1.8 +/- 0.015 nA/pF, and that of the inwardly rectifying K(+) current was 0.37 +/- 0.012 nA/pF (at 30 mM of [K(+)](o)). In order to induce neuronal differentiation of the neural stem cells, a full-length coding region of NeuroD, a neurogenic transcription factor, was transfected into HB1.F3 cells. Introduction of NeuroD induced expression of Na(+) currents with the current density of 0.042 +/- 0.011 nA/pF. The presence of two types of K(+) currents and expression of Na(+) currents induced by NeuroD appear to reflect the characteristic physiological features of human neural stem cells.

Lysophospholipids induce membrane hyperpolarization in microglia by activation of IKCa1 Ca(2+)-dependent K(+) channels

Effects of the lysophospholipids sphingosine-1-phosphate and lysophosphatidic acid were studied in cultured murine microglia using the patch-clamp and video imaging techniques. Both lysophospholipids induced transient membrane hyperpolarization and K(+) current activation. The lysophospholipid-induced K(+) current was blocked by charybdotoxin or iberiotoxin, but was unaffected by apamin. In recordings with 1 microM intracellular free Ca(2+), Ca(2+)-dependent K(+) currents of microglia showed a similar pharmacological profile to lysophospholipid-induced currents. The Ca(2+)-dependent K(+) channels activated in microglia by lysophospholipids are most likely encoded by the IKCa1 channel gene. The presence of IKCa1 mRNA in microglia was demonstrated by reverse transcriptase-polymerase chain reaction studies. Ca(2+) imaging experiments revealed increases in the intracellular free Ca(2+) concentration of microglia to a mean value of about 400 nM after application of 1 microM sphingosine-1-phosphate or 1 microM lysophosphatidic acid. We suggest that the transient membrane hyperpolarization seen in microglia following exposure to sphingosine-1-phosphate or lysophosphatidic acid is caused by activation of IKCa1 Ca(2+)-dependent K(+) channels. Increases in the concentration of intracellular free Ca(2+) evoked by the lysophospholipids are sufficient to activate microglial Ca(2+)-dependent K(+) channels.

Inhibition of store-operated Ca(2+) influx by acidic extracellular pH in cultured human microglia

The effects of extracellular acidification on Ca(2+)-dependent signaling pathways in human microglia were investigated using Ca(2+)-sensitive fluorescence microscopy. Adenosine triphosphate (ATP) was used to elicit Ca(2+) responses primarily dependent on the depletion of intracellular endoplasmic reticulum (ER) stores, while platelet-activating factor (PAF) was used to elicit responses primarily dependent on store-operated channel (SOC) influx of Ca(2+). The duration of transient responses induced by ATP was not significantly different in standard physiological pH 7.4 (mean duration 30.2 +/- 2.5 s) or acidified pH 6.2 (mean duration 31.7 +/- 2.8 s) extracellular solutions. However, the time course of the PAF response at pH 7.4 was significantly reduced by 87% with external pH at 6.2. These results suggest that acidification of extracellular solutions inhibits SOC entry of Ca(2+) with little or no effect on depletion of ER stores. Changes of extracellular pH over the range from 8.6 to 6.2 during the development of a sustained SOC influx induced by PAF resulted in instantaneous modulation of SOC amplitude indicating a rapidly reversible effect of pH on this Ca(2+) pathway. Whole-cell patch clamp recordings showed external acidification blocked depolarization-activated outward K(+) current indicating cellular depolarization may be involved in the acid pH inhibition. Since SOC mediated influx of Ca(2+) is strongly modulated by membrane potential, the electrophysiological data suggest that acidification may act to inhibit SOC by cellular depolarization. These results suggest that acidification observed during cerebral ischemia may alter microglial responses and functions.

Acute actions of tumor necrosis factor-alpha on intracellular Ca(2+) and K(+) currents in human microglia

The effects of acute application of the pro-inflammatory cytokine tumor necrosis factor-alpha (TNFalpha) on levels of intracellular Ca(2+) ([Ca(2+)]i) and on whole-cell outward and inward K(+) currents were studied in cultured human microglia. TNFalpha elicited a linear increase in [Ca(2+)]i to a plateau level in microglia bathed in either standard physiological saline solution or Ca(2+)-free physiological saline solution. The rate of increase of [Ca(2+)]i or the level of [Ca(2+)]i attained was not significantly altered in the absence of external Ca(2+) indicating that Ca(2+) influx did not contribute appreciably to the cytokine-induced rise in [Ca(2+)]i. This point was directly confirmed using Mn(2+) quenching where no change in signal fluorescence was observed with TNFalpha treatment of microglia in Ca(2+)-free physiological saline solution. The rate of increase of [Ca(2+)]i induced by TNFalpha in Ca(2+)-free physiological saline solution was not altered by prior application of ATP to deplete inositol triphosphate stores indicating that these stores did not contribute to the cytokine response. In whole-cell patch clamp recordings, the acute treatment of human microglia with TNFalpha led to the expression of an outward K(+) current in one-third (14 of 41) of cells. This current was activated at potentials positive to -30 mV, showed rapid kinetics of activation with no evident inactivation and had an I-V relation exhibiting outward rectification. Analysis of tail currents showed reversal of the outward K(+) current near -70 mV and tetraethylammonium (10 mM) inhibited the outward K(+) current to 24% of control level. Acute application of TNFalpha had no effect to alter inward rectifier currents generated from voltage ramps. The signaling pathways involving TNFalpha modulation of [Ca(2+)]i and K(+) channels in human microglia may contribute to functional and pathological actions of the cytokine in the brain.

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.

Voltage-gated proton channels in microglia

Microglia, macrophages that reside in the brain, can express at least 12 different ion channels, including voltage-gated proton channels. The properties of H+ currents in microglia are similar to those in other phagocytes. Proton currents are elicited by depolarizing the membrane potential, but activation also depends strongly on both intracellular pH (pH(i)) and extracellular pH (pH(o)). Increasing pH(o) or lowering pH(i) promotes H+ channel opening by shifting the activation threshold to more negative potentials. H+ channels in microglia open only when the pH gradient is outward, so they carry only outward current in the steady state. Time-dependent activation of H+ currents is slow, with a time constant roughly 1 s at room temperature. Microglial H+ currents are inhibited by inorganic polyvalent cations, which reduce H+ current amplitude and shift the voltage dependence of activation to more positive potentials. Cytoskeletal disruptive agents modulate H+ currents in microglia. Cytochalasin D and colchicine decrease the current density and slow the activation of H+ currents. Similar changes of H+ currents, possibly due to cytoskeletal reorganization, occur in microglia during the transformation from ameboid to ramified morphology. Phagocytes, including microglia, undergo a respiratory burst, in which NADPH oxidase releases bactericidal superoxide anions into the phagosome and stoichiometrically releases protons into the cell, tending to depolarize and acidify the cell. H+ currents may help regulate both the membrane potential and pH(i) during the respiratory burst. By compensating for the efflux of electrons and counteracting intracellular acidification, H+ channels help maintain superoxide anion production.

Potentiation of a voltage-gated proton current in acidosis-induced swelling of rat microglia

Microglia are equipped with a strong proton (H(+)) extrusion pathway, a voltage-gated H(+) channel, probably to compensate for the large amount of H(+) generated during phagocytosis; however, little is known about how this channel is regulated in pathological states. Because neural damage is often associated with intracellular and extracellular acidosis, we examined the regulatory mechanisms of the H(+) current of rat spinal microglia in acidic environments. More than 90% of round/amoeboid microglia expressed the H(+) current, which was characterized by slow activation kinetics, dependencies on both intracellular and extracellular pH, and blockage by Zn(2+). Extracellular lactoacidosis, pH 6.8, induced intracellular acidification and cell swelling. Cell swelling was also induced by intracellular dialysis with acidic pipette solutions, pH 5.5-6.8, at normal extracellular pH 7.3 in the presence of Na(+). The H(+) currents were increased in association with cell swelling as shown by shifts of the half-activation voltage to more negative potentials and by acceleration of the activation kinetics. The acidosis-induced cell swelling and the accompanying potentiation of the H(+) current required nonhydrolytic actions of intracellular ATP and were inhibited by agents affecting actin filaments (phalloidin and cytochalasin D). The H(+) current was also potentiated by swelling caused by hypotonic stress. These findings suggest that the H(+) channel of microglia can be potentiated via cell swelling induced by intracellular acidification. This potentiation might operate as a negative feedback mechanism to protect microglia from cytotoxic acidification and hence acidosis-induced swelling in pathological states of the CNS.

Temporal fluctuations of voltage-gated proton currents in rat spinal microglia via pH-dependent and -independent mechanisms

Voltage-gated proton (H(+)) channels are unique mechanisms to extrude a massive amount of H(+), and are proposed to regulate intracellular pH of microglia during respiratory bursts. Temporal variations of the H(+) current were studied in rat spinal microglia cultivated on the glial cell layer using the voltage-ramp protocol. Repetitive applications of the large and long-lasting depolarization decreased the amplitudes of the H(+) current transiently and reversibly. This decrease was accompanied by a shift of the reversal potential to a more positive direction, indicating that a drop in the transmembrane pH gradient (delta pH) by the H(+) efflux through the channel reduced the current. The decline of the H(+) current during depolarizations was also observed in a rat microglial cell line (GMI-R1). An increase in the extracellular buffer suppressed the reduction of the current, suggesting that H(+) secreted into the extracellular space contributed to the drop in delta pH. On the other hand, the amplitudes of the H(+) current often fluctuated greatly at intervals of 5-20 min without changes in delta pH. These results suggest that the H(+) current of microglia is tuned via both delta pH-dependent and -independent mechanisms, which may regulate both microglial behavior and the pH environments of the surrounding neural tissue.

Anion channels modulate store-operated calcium influx in human microglia

Recent work from this laboratory has demonstrated that purinergic-mediated depolarization of human microglia inhibited a store-operated pathway for entry of Ca2+. We have used Fura-2 spectrofluorometry to investigate the effects on store-operated Ca2+ influx induced by replacement of NaCl with Na-gluconate in extracellular solutions. Three separate procedures were used to activate store-operated channels. Platelet activating factor (PAF) was used to generate a sustained influx of Ca2+ in standard physiological saline solution (PSS). The magnitude of this response was depressed by 70% after replacement of PSS with low Cl- PSS. A second procedure used ATP, initially applied in Ca2+-free PSS solution to deplete intracellular stores. The subsequent perfusion of PSS solution containing Ca2+ resulted in a large and sustained entry of Ca2+, which was inhibited by 75% with low Cl- PSS. The SERCA inhibitor cyclopiazonic acid (CPA) was used to directly deplete stores in zero-Ca2+ PSS. Following the introduction of PSS containing Ca2+, a maintained stores-operated influx of Ca2+ was evident which was inhibited by 77% in the presence of the low Cl- PSS. Ca2+ influx was linearly reduced with cell depolarization in elevated K+ (7.5 to 35 mM) suggesting that changes in external Cl- were manifest as altered electrical driving force for Ca2+ entry. However, 50 mM external KCl effectively eliminated divalent entry which may indicate inactivation of this pathway with high magnitudes of depolarization. Patch clamp studies showed low Cl-PSS to cause depolarizing shifts in both holding currents and reversal potentials of currents activated with voltage ramps. The results demonstrate that Cl- channels play an important role in regulating store-operated entry of Ca2+ in human microglia.

"Allograft-inflammatory-factor-1 is upregulated in microglial cells in human cerebral infarctions"

Allograft inflammatory factor-1 (AIF-1) is a 17-kDa-peptide identified in rat cardiac allografts undergoing chronic rejection and in activated microglial cells in inflammatory autoimune disease of the CNS. We have investigated the expression of AIF-1 in 18 autopsy cases of human focal cerebral infarction. AIF-1-positive cells show the morphology of microglia and are CD68- but not GFAP-positive. The peptide is expressed at a low level in normal brain. In infarctions, activated microglial cells in the area of glial reaction show strongly enhanced cytoplasmic immunoreactivity. The density of AIF-1-expressing cells increases during the first three days post infarction and remains elevated until chronic cystic stages. The upregulation of AIF-1-immunoreactivity precedes the rise in expression of the S-100-protein MRP-8. We conclude that AIF-1 is a sensitive marker of human microglial activation not only in inflammation but also in non-inflammatory lesions of the CNS.

Allograft-inflammatory-factor-1 is upregulated in microglial cells in human cerebral infarctions

Allograft inflammatory factor-1 (AIF-1) is a 17-kDa-peptide identified in rat cardiac allografts undergoing chronic rejection and in activated microglial cells in inflammatory autoimune disease of the CNS. We have investigated the expression of AIF-1 in 18 autopsy cases of human focal cerebral infarction. AIF-1-positive cells show the morphology of microglia and are CD68- but not GFAP-positive. The peptide is expressed at a low level in normal brain. In infarctions, activated microglial cells in the area of glial reaction show strongly enhanced cytoplasmic immunoreactivity. The density of AIF-1-expressing cells increases during the first three days post infarction and remains elevated until chronic cystic stages. The upregulation of AIF-1-immunoreactivity precedes the rise in expression of the S-100-protein MRP-8. We conclude that AIF-1 is a sensitive marker of human microglial activation not only in inflammation but also in non-inflammatory lesions of the CNS.

Acute application of interleukin-1beta induces Ca(2+) responses in human microglia

The effects of the pro-inflammatory cytokine interleukin-1-beta (IL-1beta) on levels of intracellular calcium [Ca(2+)](i) in cultured human microglia have been studied using the fluorescent Ca(2+) indicator fura-2. IL-1beta (2 ng/ml) caused a slow, progressive increase in [Ca(2+)](i) in standard Ca(2+)-containing physiological solution (PSS). A similar effect was observed in separate studies using Ca(2+)-free PSS, however, the mean rate of increase was significantly lower than that measured with PSS. Similar results were obtained in a separate protocol, where cells were exposed to both IL-1beta in Ca(2+)-free PSS and PSS. The slope of the IL-1beta induced increase of [Ca(2+)](i) in Ca(2+)-free PSS was not altered when adenosine triphosphate was added prior to application of the cytokine. These results suggest that IL-1beta-induced responses in human microglia involve both a Ca(2+) entry pathway and a mechanism of intracellular increase other than from IP(3)-sensitive stores.

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.

Voltage-gated proton currents in microglia of distinct morphology and functional state

Whole-cell patch-clamp measurements were performed to investigate voltage-gated proton currents (I(PR)) in cultured murine microglia of distinct morphology and functional state. We studied I(PR) in ameboid microglia of untreated cultures, in ameboid microglia which had been activated by lipopolysaccharide, and in ramified microglia which had been exposed to astrocyte-conditioned medium. Proton currents of these three microglia populations did not differ regarding their activation threshold or the voltage dependence of steady-state activation. Moreover, pharmacological properties of I(PR) were similar: proton currents were sensitive to extracellularly applied Zn2+ or La3+, and could be abolished by each of those at a concentration of 100 microM. In the presence of extracellular Na+, I(PR) was decreased to a similar small extent due to activity of the Na+/H+ exchanger in all microglial populations. In contrast, proton currents of microglia differed between the three cell populations with respect to their current density and their time-course of activation: in comparison with untreated microglia, the current density of I(PR) was reduced by about 50% in microglia after their treatment with either lipopolysaccharide or astrocyte-conditioned medium. Moreover, I(PR) activated significantly more slowly in cells exposed to lipopolysaccharide or astrocyte-conditioned medium than in untreated cells. It can be concluded that the distinct H+ current characteristics of the three microglial populations do not correlate with the functional state of the cells.

Colonisation of the developing human brain and spinal cord by microglia: a review

Microglia are the immune effector cells of the nervous system. The prevailing view is that microglia are derived from circulating precursors in the blood, which originate from the bone-marrow. Colonisation of the central nervous system (CNS) by microglia is an orchestrated response during human fetal development related to the maturation of the nervous system. It coincides with vascularisation, formation of radial glia, neuronal migration and myelination primarily in the 4th-5th months and beyond. Microglial influx generally conforms to a route following white matter tracts to gray areas. We have observed that colonisation of the spinal cord begins around 9 weeks, with the major influx and distribution of microglia commencing around 16 weeks. In the cerebrum, colonisation is in progress during the second trimester, and ramified microglial forms are widely distributed within the intermediate zone by the first half of intra-uterine life (20-22 weeks). A distinct pattern of migration occurs along radial glia, white matter tracts and vasculature. The distribution of these cells is likely to be co-ordinated by spatially and temporally regulated, anatomical expression of chemokines including RANTES and MCP-1 in the cortex; by ICAM-2 and PECAM on radiating cerebral vessels and on capillaries within the germinal layer, and apoptotic cell death overlying this region. The phenotype and functional characteristics of fetal microglia are also outlined in this review. The need for specific cellular interactions and targeting is greater within the central nervous system than in other tissues. In this respect, microglia may additionally contribute towards CNS histogenesis.

Effects of ATP and elevated K+ on K+ currents and intracellular Ca2+ in human microglia

We have used whole-cell patch-clamp recordings and calcium microfluorescence measurements to study the effects of ATP and elevated external K+ on properties of human microglia. The application of ATP (at 0.1 mM) led to the activation of a transient inward non-selective cationic current at a cell holding potential of -60 mV and a delayed, transient expression of an outward K+ current activated with depolarizing steps applied from holding level. The ATP response included an increase in inward K+ conductance and a depolarizing shift in reversal potential as determined using a voltage ramp waveform applied from -120 to -50 mV. Fura-2 microspectrofluorescence measurements showed intracellular calcium to be increased following the application of ATP. This response was characterized by an initial transient phase, which persisted in Ca2+-free media and was due to release of Ca2+ from intracellular storage sites. The response had a later plateau phase, consistent with Ca2+ influx. In addition, ATP-induced changes in intracellular Ca2+ exhibited prominent desensitization. Elevated external K+ (at 40 mM) increased inward K+ conductance and shifted the reversal potential in the depolarizing direction, with no effect on outward K+ current or the level of internal Ca2+. The results of these experiments show the differential responses of human microglia to ATP and elevated K+, two putative factors associated with neuronal damage in the central nervous system.

Involvement of stretch-activated Cl- channels in ramification of murine microglia

A stretch-activated Cl- current (ICl) was investigated in cultured murine microglia using the whole-cell configuration of the patch-clamp technique. After application of membrane stretch, a Cl- current appeared within seconds, and its amplitude increased further within 3-8 min. ICl underwent rundown, which was prevented by addition of 4 mM ATP to the intracellular perfusing solution. The stretch-activated Cl- current exhibited outward rectification and did not show any voltage-dependent gating. Lowering the concentration of extracellular Cl- from 142 to 12 mM by equimolar substitution of Cl- with gluconate shifted the reversal potential of ICl by 41.6 +/- 1.8 mV in the depolarizing direction. 4, 4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) blocked ICl in a voltage- and time-dependent manner. At a test potential of +40 mV, a half-maximal blockade at 16.1 microM DIDS and at 71.0 microM SITS was determined for ICl. At a concentration of 200 microM, 5-nitro-2-(3-phenylpropylamino)benzoic acid or flufenamic acid blocked ICl by 88% and 75%, respectively. Each of these four Cl- channel blockers reversibly inhibited the ramification process of microglia, whereas blockers of voltage-gated Na+ and K+ channels did not affect the transformation of microglia from their ameboid into the ramified phenotype. It is suggested that in microglia functional stretch-activated Cl- channels are required for the induction of ramification but not for maintaining the ramified shape.