Effects of phorbol ester on intracellular Ca2+ and membrane currents in cultured human microglia

Aripiprazole inhibits superoxide generation from phorbol-myristate-acetate (PMA)-stimulated microglia in vitro: implication for antioxidative psychotropic actions via microglia

Altered antioxidant status has been implicated in schizophrenia. Microglia, major sources of free radicals such as superoxide (•O(2)(-)), play crucial roles in various brain pathologies. Recent postmortem and imaging studies have indicated microglial activation in the brain of schizophrenic patients. We previously demonstrated that atypical antipsychotics including aripiprazole significantly inhibited the release of nitric oxide and proinflammatory cytokines from interferon-γ-stimulated microglia in vitro. Antioxidative effects of antipsychotics via modulating microglial superoxide generation have never been reported. Therefore, we herein investigated the effects of antipsychotics on the •O(2)(-) generation from phorbol-myristate-acetate (PMA)-stimulated rodent microglia by the electron spin resonance (ESR) spectroscopy and also examined the intracellular mechanism by intracellular Ca(2+) imaging and immunostaining. Neuronal damage induced by microglial activation was also investigated by the co-culture experiment. Among various antipsychotics, only aripiprazole inhibited the •O(2)(-) generation from PMA-stimulated microglia. Aripiprazole proved to inhibit the •O(2)(-) generation through the cascade of protein kinase C (PKC) activation, intracellular Ca(2+) regulation and NADPH oxidase activation via cytosolic p47(phox) translocation to the plasma/phagosomal membranes. Formation of neuritic beading, induced by PMA-stimulated microglia, was attenuated by pretreatment of aripiprazole. D2R antagonism has long been considered as the primary therapeutic action for schizophrenia. Aripiprazole with D2R partial agonism is effective like other antipsychotics with fewer side effects, while aripiprazole's therapeutic mechanism itself remains unclear. Our results imply that aripiprazole may have psychotropic effects by reducing the microglial oxidative reactions and following neuronal reactions, which puts forward a novel therapeutic hypothesis in schizophrenia research.

Investigations with cultured human microglia on pathogenic mechanisms of Alzheimer's disease and other neurodegenerative diseases

Inflammation-mediated mechanisms for human neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) have evolved from being on the fringe of medical hypotheses to mainstream thinking. Pioneering immunopathology studies with human brain tissues identified microglia associated with neuropathologic hallmarks of these diseases. As activated macrophages were known to produce many potential toxic products, this gave rise to the hypothesis that activated microglia (brain resident macrophages) could be contributing to the degeneration of key target neurons in these diseases, as well as potential vascular dysfunction. Studies with microglia derived from different sources, including human brains, have confirmed that activated microglia can mediate neuronal cell death. Based on these theories, a number of human clinical trials with antiinflammatory agents have been carried out on AD patients. Results to date have indicated a lack of effectiveness at slowing disease progression and have begun to cast doubt on the significance of inflammation in AD. It has been shown recently that activating microglia through immunization of amyloid plaque-developing mice with amyloid beta peptide (Abeta) has promise as a therapeutic strategy and despite some setbacks, has potential as a treatment for AD patients. This article will consider experimental data with microglia to determine whether the additional targets need to be investigated. The use of human microglia cultures, in particular those derived from elderly diseased human brains, offers an experimental system that can closely model the cell type activated in human neurodegenerative diseases. Experimental data produced by our laboratory and others is reviewed to determine the contribution of this unique experimental model to understanding disease mechanisms and possibly discovering new therapeutic targets.

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.

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.

Induction of protein kinase Cdelta subspecies in neurons and microglia after transient global brain ischemia

The delayed death of CA1 neurons after global brain ischemia is associated with induction of apoptosis genes and is inhibited by protein synthesis inhibitors, suggesting that the degeneration of CA1 pyramidal neurons is an active process that requires new gene expression. The transient global ischemia model has been extensively used to identify enzymes and other proteins underlying delayed neuronal cell death. The expression of protein kinase C (PKC) subspecies after 20 minutes of global brain ischemia produced by a four-vessel occlusion model in the rat was studied. From the multiple PKC subspecies studied, only PKCdelta mRNA was significantly up-regulated in CA1 pyramidal neurons at 24 hours and in activated microglia at 3 to at least 7 days after ischemia. The induction of PKCdelta mRNA was also found in the cortex at 8 hours and 3 days after ischemia. This cortical but not hippocampal induction was regulated by an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptor antagonist, 6-nitro-7-sulfamobenzo[f]quinoxaline-2,3-dione, and glucocorticoids. An N-methyl-D-aspartate receptor antagonist, MK-801, was without effect on the induction of PKCdelta subspecies. The selective and prolonged induction of the PKCdelta mRNA and protein first in CA1 pyramidal neurons and at a later stage in activated microglia suggests that the PKCdelta isozyme may take part in regulation of the delayed death of CA1 neurons after transient global brain ischemia.

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.

Process extension and intracellular Ca2+ in cultured murine oligodendrocytes

Previous investigations have shown that phorbol esters stimulate process extension in oligodendrocytes (OL), likely by the activation of protein kinase C (PKC). In this report, we demonstrate that treatment of OL with 4beta-phorbol-12, 13-dibutyrate (PDB; 0.1-1 microM) resulted in an increase in intracellular Ca2+ concentration ([Ca2+]i) from 94+/-2 nM (mean+/-S.E.M.) to 244+/-10 nM. This increase was produced by Ca2+ influx through a La3+-insensitive pathway. Changes in [Ca2+]i were also produced by modifying the extracellular Ca2+ concentration ([Ca2+]o) where [Ca2+]i was increased by elevations in [Ca2+]o. In parallel experiments we found that increased [Ca2+]o alone, without concurrent phorbol ester application, resulted in increased OL process extension as determined by the percent of OL with long processes (greater than 3 times the cell body diameter). These results demonstrate that increasing [Ca2+]o stimulates OL process outgrowth. Furthermore, both elevations in [Ca2+]o and PDB exposure increase [Ca2+]i, suggesting that some of the effects of phorbol esters on OL process extension are likely mediated by changes in [Ca2+]i.

Cholinergic agonists increase intracellular Ca2+ in cultured human microglia

Microglia are resident phagocytic cells in the central nervous system (CNS), and can be activated in response to various stimuli including neurotransmitters. Using fura-2 imaging, we investigated the effects of carbachol (CCh), a cholinergic agonist, on [Ca2+]i in cultured human microglia. Treatment of microglia with CCh (100 microM) produced a transient increase in [Ca2+]i, which was atropine-sensitive and was associated with release from intracellular Ca2+ stores. Successive applications of CCh showed a change in the amplitude of the [Ca2+]i signal consistent with desensitization. These results show that human microglia express functional muscarinic receptors and respond to cholinergic agonists. The rapid change of [Ca2+]i in microglia may serve as a second messenger to trigger downstream cascades which contribute to signalling pathways in CNS pathology.

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.