Modulation of membrane potential by extracellular pH in activated microglia in rats

Kv1.3 inhibition attenuates neuroinflammation through disruption of microglial calcium signaling

In the last 5 years inhibitors of the potassium channel KV1.3 have been shown to reduce neuroinflammation in rodent models of ischemic stroke, Alzheimer's disease, Parkinson's disease and traumatic brain injury. At the systemic level these beneficial actions are mediated by a reduction in microglia activation and a suppression of pro-inflammatory cytokine and nitric oxide production. However, the molecular mechanisms for the suppressive action of KV1.3 blockers on pro-inflammatory microglia functions was not known until our group recently demonstrated that KV1.3 channels not only regulate membrane potential, as would be expected of a voltage-gated potassium channel, but also play a crucial role in enabling microglia to resist depolarizations produced by the danger signal ATP thus regulating calcium influx through P2X4 receptors. We here review the role of KV1.3 in microglial signaling and show that, similarly to their role in T cells, KV1.3 channels also regulated store-operated calcium influx in microglia.

Activation of microglial N-methyl-D-aspartate receptors triggers inflammation and neuronal cell death in the developing and mature brain

OBJECTIVE

Activated microglia play a central role in the inflammatory and excitotoxic component of various acute and chronic neurological disorders. However, the mechanisms leading to their activation in the latter context are poorly understood, particularly the involvement of N-methyl-D-aspartate receptors (NMDARs), which are critical for excitotoxicity in neurons. We hypothesized that microglia express functional NMDARs and that their activation would trigger neuronal cell death in the brain by modulating inflammation.

METHODS AND RESULTS

We demonstrate that microglia express NMDARs in the murine and human central nervous system and that these receptors are functional in vitro. We show that NMDAR stimulation triggers microglia activation in vitro and secretion of factors that induce cell death of cortical neurons. These damaged neurons are further shown to activate microglial NMDARs and trigger a release of neurotoxic factors from microglia in vitro, indicating that microglia can signal back to neurons and possibly induce, aggravate, and/or maintain neurologic disease. Neuronal cell death was significantly reduced through pharmacological inhibition or genetically induced loss of function of the microglial NMDARs. We generated Nr1 LoxP(+/+) LysM Cre(+/-) mice lacking the NMDAR subunit NR1 in cells of the myeloid lineage. In this model, we further demonstrate that a loss of function of the essential NMDAR subunit NR1 protects from excitotoxic neuronal cell death in vivo and from traumatic brain injury.

INTERPRETATION

Our findings link inflammation and excitotoxicity in a potential vicious circle and indicate that an activation of the microglial NMDARs plays a pivotal role in neuronal cell death in the perinatal and adult 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.

Peripheral osmotic stimulation inhibits the brain's innate immune response to microdialysis of acidic perfusion fluid adjacent to supraoptic nucleus

During the brain's innate immune response microglia, astroglia and ependymal cells resolve/repair damaged tissue and control infection. Released interleukin-1beta (IL-1beta) reaching cerebroventricles stimulates circumventricular organs (CVOs; subfornical organ, SFO; organum vasculosum lamina terminalis, OVLT), the median preoptic nucleus (MePO), and magnocellular and parvocellular neurons in the supraoptic (SON) and paraventricular (PVN) nuclei. Hypertonic saline (HS) also activates these osmosensory CVOs and neuroendocrine systems, but, in contrast to IL-1beta, inhibits the peripheral immune response. To examine whether the brain's innate immune response is attenuated by osmotic stimulation, sterile acidic perfusion fluid was microdialyzed (2 microl/min) in the SON area of conscious rats for 6 h with sterile HS (1.5 M NaCl) injected subcutaneously (15 ml/kg) at 5 h. Immunohistochemistry identified cytokine sources (IL-1beta(+); OX-42(+) microglia) and targets (IL-1R(+); inducible cyclooxygenase, COX-2(+); c-Fos(+)) near the probe, in CVOs, MePO, ependymal cells, periventricular hypothalamus, SON, and PVN. Inserting the probe stimulated magnocellular neurons (c-Fos(+); SON; PVN) via the MePO (c-Fos(+)), a response enhanced by HS. Microdialysis activated microglia (OX-42(+); amoeboid/hypertrophied; IL-1beta(+)) in the adjacent SON and bilaterally in perivascular areas of the PVN, periventricular hypothalamus and ependyma, coincident with c-Fos expression in ependymal cells and COX-2 in the vasculature. These microglial responses were attenuated by HS, coincident with activating parvocellular and magnocellular neuroendocrine systems and elevating circulating IL-1beta, oxytocin, and vasopressin. Acidosis-induced cellular injury from microdialysis activated the brain's innate immune response by a mechanism inhibited by peripheral osmotic stimulation.

Physiology of microglial cells

Microglial cells in culture and in situ express a defined pattern of K(+) channels, which is distinct from that of other glial cells and neurons. This pattern undergoes defined changes with microglial activation. As expected for a cell with immunological properties, microglia express a variety of cytokine and chemokine receptors, which are linked to the mobilization of Ca(2+) (cytosolic free calcium) from internal stores. Microglial cells also have the capacity to respond to neuronal activity: they express receptors for the major excitatory receptor glutamate and the main inhibitory receptor GABA (gamma-amino butyric acid). By expressing purinergic receptors, microglia can sense astrocyte activity in the form of Ca(2+) waves. Activation of transmitter receptors can affect cytokine release which is a potential means as to how brain activity can affect immune function.

pH dependent uptake of porphyrin-type photosensitizers by solid tumor cells in vitro is not induced by modification of transmembrane potential

The uptake of HpIX, TPPS2a and mTHPC by WiDr, THX cells and skin fibroblasts at pH 7.4 and 6.8 was compared. In the absence of serum, the uptake of HpIX was higher at lower pH. The difference was significant in WiDr cells (P < 0.01) and skin fibroblasts (P < 0.05). TPPS2a nor mTHPC showed any pH dependent uptake. Lowering the extracellular pH resulted in a significant depolarization (3-8 mV) of the cells. Application of tetraethylammonium chloride did not affect the cellular uptake of any of the photosensitizers. We conclude that the pH dependent uptake of photosensitizers is not mainly related to altered transmembrane potential.

Effects of ELF magnetic field on membrane protein structure of living HeLa cells studied by Fourier transform infrared spectroscopy

The effects of exposure to a 50 Hz magnetic field (maximum of 41.7 to 43.6 mT) on the membrane protein structures of living HeLa cells were studied using attenuated total reflection infrared spectroscopy. One min of such exposure shifted peak absorbance of the amide I band to a smaller wave number, reduced peak absorbance of the amide II band, and increased absorbance at around 1600 cm(-1). These results suggest that exposure to the ELF magnetic field has reversible effects on the N-H inplane bending and C-N stretching vibrations of peptide linkages, and changes the secondary structures of alpha-helix and beta-sheet in cell membrane proteins.

pH, serum proteins and ionic strength influence the uptake of merocyanine 540 by WiDr cells and its interaction with membrane structures

It has been suggested that selective uptake of photosensitizers is due to significantly lower pH of the interstitial fluid in tumors compared to normal tissue. Therefore, the cellular uptake of merocyanine 540 (MC 540) was examined at two pH values: 6.8+/-0.1 and 7.4+/-0.1. There was no difference in spectral properties (absorption and fluorescence maxima positions, fluorescence intensity) of the drug in the presence of increasing amounts of either human blood plasma or FCS (0-2%) at the two pH values investigated. Nevertheless, significantly higher amounts of the drug were taken up by WiDr cells at pH 6.8+/-0.1, both in the presence of 10% FCS and in the absence of FCS. The absorption spectra of MC 540 in the presence of egg phosphatidylcholine (PC) liposomes turned out to be NaCl concentration-dependent (0.00-0.30 mol l(-1)). Membrane fluidity, as measured by fluorescence anisotropy of diphenylhexatriene (DPH), was unchanged within the experimental error in the NaCl concentration range 0.01-0.30 mol l(-1). The spectral changes indicated an enhancement of the incorporation of MC 540 into lipid membranes with increasing ionic strength. Such a salt concentration dependence suggests a possible involvement of the surface potential in the interaction of MC 540 with lipid membranes. The results might provide an explanation of the pH dependency of the cellular uptake of MC 540 observed in this study.

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.

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.

Beta-amyloid peptide induces the expression of voltage dependent outward rectifying K+ channels in rat microglia

Upregulation of voltage-dependent outward rectifying K+ (Kv) channels has been reported in activated microglia. Since beta-amyloid peptide (A beta) is known to activate microglia, we tested whether the exposure of cultured rat microglia to A beta fragment 25-35 (A beta 25-35) induced the Kv current. A beta 25-35 in 5-200 nM concentration range significantly increased Kv current density, while there was small change in inward rectifying K+ current density. The full length A beta peptide (A beta 1-42) also increased Kv current. However, the control peptide, A beta 35-25, did not induce Kv current. Most of the Kv current induced by A beta was specifically blocked by the presence of antisense deoxyoligonucleotides against Kv1.3, and Kv1.5. Thus, it is concluded that we have identified Kv1.3 and Kv1.5 as the channel types expressed in A beta-treated microglia.

Spectral imaging of MC540 during murine and human colon carcinoma cell differentiation

We studied the staining pattern of merocyanine 540 (MC540) by spectral imaging of murine CT26 and human HT29 colon carcinoma cells incubated with the dye MC540. This dye, usually considered a potential membrane probe, localized mainly in the cytoplasmic vesicles of the colon carcinoma cells. However, in cells incubated in an environment similar to that of a tumor (pH 6.7), high fluorescence was detected in the nuclear membrane and nucleoli. Under these acidic conditions, resembling the Krebs effect, a population of CT26 cells displayed fluorescent plasma membranes. In differentiating cells, exhibiting cell cycle arrest at G(1)-phase and an elevated level of alkaline phosphatase, MC540 fluorescence was confined to cytoplasmic vesicles and was not detected in the plasma membrane or in the nucleoli. Cell sorting analysis of both cell types at pH 5.0 revealed higher fluorescence intensity in proliferating cells compared to differentiating cells. The fluorescence intensity of MC540-stained cells reached a maximum at pH 5.0, although the fluorescence of MC540 dye was maximal at pH 7.2. This phenomenon may result from increased binding of MC540 monomers to the cells because disaggregation of the dye with Triton X-100 produced similar results. We conclude that nucleolar localization of MC540 and an elevated fluorescence intensity can be used as indicators for proliferating cells in the characteristically acidic tumor environment. (J Histochem Cytochem 49:147-153, 2001)

Inward and outward rectifying potassium currents set membrane potentials in activated rat microglia

Activation of cultured rat microglial cells with lipopolysaccharide (LPS), induced outward rectifying K+ (K(V)) current in addition to already existing inward rectifying K+ current (K(IR)). By measuring zero-current membrane-potentials using whole-cell patch-clamp method, we showed that K(V) current plays a direct role in setting membrane potential to near -45 mV. Since the membrane potentials of microglia show two prominent peaks at -45 and -70 mV, we hypothesize that K(IR) current might set the membrane potential to near -70 mV. We observed that cells with larger K(IR) current had a zero-current membrane-potential at around -70 mV, and that blocking of K(IR) current with Ba2+ depolarized membrane potentials to near -45 mV. These results indicate that the amounts of K(IR), and K(V) current determine the zero-current membrane-potentials in LPS-activated microglia.