mRNA coding for neurotransmitter receptors in a human astrocytoma

M2 muscarinic receptors negatively modulate cell migration in human glioblastoma cells

Glioblastoma (GB) is a very aggressive human brain tumor. The high growth potential and invasiveness make this tumor surgically and pharmacologically untreatable. Our previous work demonstrated that the activation of the M2 muscarinic acetylcholine receptors (M2 mAChRs) inhibited cell proliferation and survival in GB cell lines and in the cancer stem cells derived from human biopsies. The aim of the present study was to investigate the ability of M2 mAChR to modulate cell migration in two different GB cell lines: U87 and U251. By wound healing assay and single cell migration analysis performed by time-lapse microscopy, we demonstrated the ability of M2 mAChRs to negatively modulate cell migration in U251 but not in the U87 cell line. In order to explain the different effects observed in the two cell lines we have evaluated the possible involvement of the intermediate conductance calcium-activated potassium (IKCa) channel. IKCa channel is present in the GB cells, and it has been demonstrated to modulate cell migration. Using the perforated patch-clamp technique we have found that selective activation of M2 mAChR significantly reduced functional density of the IKCa current in U251 but not in U87 cells. To understand whether the M2 mAChR mediated reduction of ion channel density in the U251 cell line was relevant for the cell migration impairment, we tested the effects of TRAM-34, a selective inhibitor of the IKCa channel, in wound healing assay. We found that it was able to markedly reduce U251 cell migration and significantly decrease the number of invadopodia-like structure formations. These results suggest that only in U251 cells the reduced cell migration M2 mAChR-mediated might involve, at least in part, the IKCa channel.

M2 Muscarinic Receptor Activation Impairs Mitotic Progression and Bipolar Mitotic Spindle Formation in Human Glioblastoma Cell Lines

Background: Glioblastoma multiforme (GBM) is characterized by several genetic abnormalities, leading to cell cycle deregulation and abnormal mitosis caused by a defective checkpoint. We previously demonstrated that arecaidine propargyl ester (APE), an orthosteric agonist of M2 muscarinic acetylcholine receptors (mAChRs), arrests the cell cycle of glioblastoma (GB) cells, reducing their survival. The aim of this work was to better characterize the molecular mechanisms responsible for this cell cycle arrest. Methods: The arrest of cell proliferation was evaluated by flow cytometry analysis. Using immunocytochemistry and time-lapse analysis, the percentage of abnormal mitosis and aberrant mitotic spindles were assessed in both cell lines. Western blot analysis was used to evaluate the modulation of Sirtuin2 and acetylated tubulin—factors involved in the control of cell cycle progression. Results: APE treatment caused arrest in the M phase, as indicated by the increase in p-HH3 (ser10)-positive cells. By immunocytochemistry, we found a significant increase in abnormal mitoses and multipolar mitotic spindle formation after APE treatment. Time-lapse analysis confirmed that the APE-treated GB cells were unable to correctly complete the mitosis. The modulated expression of SIRT2 and acetylated tubulin in APE-treated cells provides new insights into the mechanisms of altered mitotic progression in both GB cell lines. Conclusions: Our data show that the M2 agonist increases aberrant mitosis in GB cell lines. These results strengthen the idea of considering M2 acetylcholine receptors a novel promising therapeutic target for the glioblastoma treatment.

SIRT4 prevents excitotoxicity via modulating glutamate metabolism in glioma cells

Excitotoxicity is the presence of excessive glutamate, which is normally taken up by glutamate transporters on astrocytes. Glutamate transporter 1 (GLT-1) is the major transporter on glia cells clearing more than 90% of the glutamate. Sirtuin 4 (SIRT4) is a mitochondrial sirtuin which is expressed in the brain. Previously, it was shown that loss of SIRT4 leads to a more severe reaction to kainic acid, an excitotoxic agent, and also decreased GLT-1 expression in the brain. In this study, we aimed to investigate whether overexpression of SIRT4 is protective against excitotoxicity in glia cells. We overexpressed SIRT4 in A172 glioma cell line and treated with kainic acid in order to induce excitotoxicity. We observed that SIRT4 overexpression increased the cell viability after kainic acid treatment. In addition, reduced glutamate was detected in glutamate assay with overexpression of SIRT4 after kainic acid treatment since SIRT4 decreased cell death by preventing excitotoxicity. Our results show that overexpression of SIRT4 increased the protein levels of GLT-1 and glutamate dehydrogenase (GDH) after kainic acid (KA) treatment so that excess glutamate can be absorbed. However, overexpression of SIRT4 decreased glutamine synthetase (GS) levels. These results demonstrate that, by inhibiting GS, SIRT4 prevents glutamine formation, which will be converted to glutamate in neurons. SIRT4 prevents excitotoxicity via upregulating glutamate metabolism. Finally, our results may show that SIRT4 might prevent excitotoxicity and related cell death via reducing GS expression and upregulating GLT-1 and GDH levels. Therefore, it is important to develop therapeutics against excitotoxicity through SIRT4-related pathways in the cell.

Targeting nitric oxide and NMDA receptor-associated pathways in treatment of high grade glial tumors. Hypotheses for nitro-memantine and nitrones

Glioblastoma multiforme (GBM) is a devastating brain cancer with no curative treatment. Targeting Nitric Oxide (NO) and glutamatergic pathways may help as adjunctive treatments in GBM. NO at low doses promotes tumorigenesis, while at higher levels (above 300 nM) triggers apoptosis. Gliomas actively secrete high amounts of glutamate which activates EGR signaling and mediates degradation of peritumoral tissues via excitotoxic injury. Memantine inhibits NMDA-subtype of glutamate receptors (NMDARs) and induces autophagic death of glioma cells in vitro and blocks glioma growth in vivo. Nitro-memantines may exert further benefits by limiting NMDAR signaling and by delivery of NO to the areas of excessive NMDAR activity leading NO-accumulation at tumoricidal levels within gliomas. Due to the duality of NO in tumorigenesis, agents which attenuate NO levels may also act beneficial in treatment of GBM. Nitrone compounds including N-tert-Butyl-α-phenylnitrone (PBN) and its disulfonyl-phenyl derivative, OKN-007 suppress free radical formation in experimental cerebral ischemia. OKN-007 failed to show clinical efficacy in stroke, but trials demonstrated its high biosafety in humans including elderly subjects. PBN inhibits the signaling pathways of NF-κB, inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX). In animal models of liver cancer and glioblastoma, OKN-007 seemed more efficient than PBN in suppression of cell proliferation, microvascular density and in induction of apoptosis. OKN-007 also inhibits SULF2 enzyme, which promotes tumor growth via versatile pathways. We assume that nitromemantines may be more beneficial concomitant with chemo-radiotherapy while nitrones alone may act useful in suppressing basal tumor growth and angiogenesis.

Inwardly Rectifying K+ Currents in Cultured Oligodendrocytes from Rat Optic Nerve are Insensitive to pH

Inwardly rectifying K⁺ (Kir) channel expression signals at an advanced stage of maturation during oligodendroglial differentiation. Knocking down their expression halts the generation of myelin and produces severe abnormalities in the central nervous system. Kir4.1 is the main subunit involved in the tetrameric structure of Kir channels in glial cells; however, the precise composition of Kir channels expressed in oligodendrocytes (OLs) remains partially unknown, as participation of other subunits has been proposed. Kir channels are sensitive to H⁺; thus, intracellular acidification produces Kir current inhibition. Since Kir subunits have differential sensitivity to H⁺, we studied the effect of intracellular acidification on Kir currents expressed in cultured OLs derived from optic nerves of 12-day-old rats. Unexpectedly, Kir currents in OLs (2-4 DIV) did not change within the pH range of 8.0-5.0, as observed when using standard whole-cell voltage-clamp recording or when preserving cytoplasmic components with the perforated patch-clamp technique. In contrast, low pH inhibited astrocyte Kir currents, which was consistent with the involvement of the Kir4.1 subunit. The H⁺-insensitivity expressed in OL Kir channels was not intrinsic because Kir cloning showed no difference in the sequence reported for the Kir4.1, Kir2.1, or Kir5.1 subunits. Moreover, when Kir channels were heterologously expressed in Xenopus oocytes they behaved as expected in their general properties and sensitivity to H⁺. It is therefore concluded that Kir channel H⁺-sensitivity in OLs is modulated through an extrinsic mechanism, probably by association with a modulatory component or by posttranslational modifications.

Glycine receptors in nervous tissue and their functional role

The literature data on glycine metabolism in neural tissue, mitochondrial Gly-cleaving system, Gly-catching system in neural and glial cells are summarized. The peculiarities of localization and distribution of specific glycine receptors and binding-sites in nervous tissue of mammals are described. Four types of glycine-binding receptors are described: own specific glycine receptor (Gly-R), ionotropic receptor, which binds N-methyl-D-aspartate selectively (NMDA-R), and ionotropic receptors of g-aminobutyrate (GABA A -R, GABA С -R). The feutures of glycine effects in neuroglial cultures are discussed

Glutamate transporters in the biology of malignant gliomas

Malignant gliomas are relentless tumors that offer a dismal clinical prognosis. They develop many biological advantages that allow them to grow and survive in the unique environment of the brain. The glutamate transporters system x c (-) and excitatory amino acid transporters (EAAT) are emerging as key players in the biology and malignancy of these tumors. Gliomas manipulate glutamate transporter expression and function to alter glutamate homeostasis in the brain, which supports their own growth, invasion, and survival. As a consequence, malignant cells are able to quickly destroy and invade surrounding normal brain. Recent findings are painting a larger picture of these transporters in glioma biology, and as such are providing opportunities for clinical intervention for patients. This review will detail the current understanding of glutamate transporters in the biology of malignant gliomas and highlight some of the unique aspects of these tumors that make them so devastating and difficult to treat.

Overexpression of calcium-permeable glutamate receptors in glioblastoma derived brain tumor initiating cells

Glioblastoma multiforme is the most malignant type of primary brain tumor with a poor prognosis. These tumors consist of a heterogeneous population of malignant cells, including well-differentiated tumor cells and less differentiated cells with stem cell properties. These cancer stem cells, known as brain tumor initiating cells, likely contribute to glioma recurrence, as they are highly invasive, mobile, resistant to radiation and chemotherapy, and have the capacity to self-renew. Glioblastoma tumor cells release excitotoxic levels of glutamate, which may be a key process in the death of peritumoral neurons, formation of necrosis, local inflammation, and glioma-related seizures. Moreover, elevated glutamate levels in the tumor may act in paracrine and autocrine manner to activate glutamate receptors on glioblastoma tumor cells, resulting in proliferation and invasion. Using a previously described culturing condition that selectively promotes the growth of brain tumor initiating cells, which express the stem cell markers nestin and SOX-2, we characterize the expression of α-amino-3-hydroxy-5-methyl-4-isozolepropionic acid (AMPA)-type glutamate receptor subunits in brain tumor initiating cells derived from glioblastomas. Here we show for the first time that glioblastoma brain tumor initiating cells express high concentrations of functional calcium-permeable AMPA receptors, compared to the differentiated tumor cultures consisting of non-stem cells. Up-regulated calcium-permeable AMPA receptor expression was confirmed by immunoblotting, immunocytochemistry, and intracellular calcium imaging in response to specific agonists. Our findings raise the possibility that glutamate secretion in the GBM tumor microenvironment may stimulate brain tumor derived cancer stem cells.

CREB regulates AChE-R-induced proliferation of human glioblastoma cells

The cyclic adenosine monophosphate (AMP) response element-binding protein, CREB, often modulates stress responses. Here, we report that CREB suppresses the glioblastoma proliferative effect of the stress-induced acetylcholinesterase variant, AChE-R. In human U87MG glioblastoma cells, AChE-R formed a triple complex with protein kinase C (PKC) epsilon and the scaffold protein RACK1, enhanced PKCepsilon phosphorylation, and facilitated BrdU incorporation. Either overexpressed CREB, or antisense destruction of AChE-R mRNA, PKC, or protein kinase A (PKA) inhibitors-but not CREB combined with PKC inhibition suppressed-this proliferation, suggesting that CREB's repression of this process involves a PKC-mediated pathway, whereas impaired CREB regulation allows AChE-R-induced, PKA-mediated proliferation of glioblastoma tumors.

Expression of human epileptic temporal lobe neurotransmitter receptors in Xenopus oocytes: An innovative approach to study epilepsy

Poly(A(+)) RNA was extracted from the temporal lobe (TL) of medically intractable epileptic patients which underwent surgical TL resection. Injection of this mRNA into Xenopus oocytes led to the expression of ionotropic receptors for gamma-aminobutyric acid (GABA), kainate (KAI) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Membrane currents elicited by GABA inverted polarity at -15 mV, close to the oocyte's chloride equilibrium potential, were inhibited by bicuculline, and were potentiated by pentobarbital and flunitrazepam. These basic characteristics were also displayed by GABA currents elicited in oocytes injected with mRNAs isolated from human TL glioma (TLG) or from mouse TL. However, the GABA receptors expressed by the epileptic TL mRNA exhibited some unusual properties, consisting in a rapid current run-down after repetitive GABA applications and a large EC(50) (125 microM). AMPA alone evoked very small or nil currents, whereas KAI induced larger currents. Nevertheless, upon cyclothiazide treatment, AMPA elicited substantial currents that, like the KAI currents, were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Furthermore, the glutamate receptor 5 (GluR5) agonist, ATPA, failed to evoke an obvious current although both RT-PCR and Western blot analyses showed GluR5 expression in the epileptic TL. Oocytes injected with mouse TL or human TLG mRNAs generated KAI and AMPA currents similar to those evoked in oocytes injected with epileptic TL mRNA but, in contrast to these, the mouse TL and human TLG oocytes were also responsive to ATPA. Our findings are in accord with the concept that both a depression of GABA inhibition and a dysfunction of the KAI-receptor system maintain a high neuronal excitability that results in epileptic seizures.

Glutamate receptor activation can trigger electrical activity in human glioma cells

Cells from major types of gliomas, i.e. oligodendrogliomas and glioblastomas, are able to generate action potentials upon a current injection similar to neurons (Patt et al. (1996) Neuroscience, 71, 601-611; Labrakakis et al. (1997b) J. Neuropath. Exp. Neurol., 56, 243-254. Here, we report that activation of ionotropic glutamate receptors by the selective agonist, kainate, or by glutamate itself, depolarized the tumour cells in culture and living slices from tumour tissue, and can elicit volleys of action potentials, as recorded with the patch-clamp technique. Sixty-six percent of the glioblastoma cells, 44% of the astocytoma and 86% of the oligodendroglioma cells responded to glutamate and the specific agonist of AMPA/kainate receptors, kainate. The involvement of non-NMDA (N-methyl-D-aspartate) receptors is further supported by the observation that both kainate and glutamate currents were blocked by CNQX (6-cyano-7-nitroquinoxaline-2,3-dione). The receptor activation was accompanied by an increase in cytosolic Ca2+, as recorded with a fura-2 microfluorometric system. The Ca2+ elevation was mediated by the activation of Ca2+ channels due to membrane depolarization. The presence of voltage-gated Ca2+ channels was confirmed by patch-clamp experiments. Taken together, these findings imply that the electrophysiological properties of glioma cells are more reminiscent of those of neurons than of glial cells.

Electrophysiological properties of human astrocytic tumor cells In situ: enigma of spiking glial cells

To better understand physiological changes that accompany the neoplastic transition of astrocytes to become astrocytoma cells, we studied biopsies of low-grade, pilocytic astrocytomas. This group of tumors is most prevalent in children and the tumor cells maintain most antigenic features typical of astrocytes. Astrocytoma cells were studied with the use of whole cell patch-clamp recordings in acute biopsy slices from 4-mo- to 14-yr-old pediatric patients. Recordings from 53 cells in six cases of low-grade astrocytomas were compared to either noncancerous peritumoral astrocytes or astrocytes obtained from other surgeries. Astrocytoma cells almost exclusively displayed slowly activating, sustained, tetraethylammonium (TEA)-sensitive outward potassium currents (delayed rectifying potassium currents; IDR) and transient, tetrodotoxin (TTX)-sensitive sodium currents (INa). By contrast, comparison glial cells from peritumoral regions or other surgeries showed IDR and INa, but in addition these cells also expressed transient "A"-type K+ currents and inwardly rectifying K+ currents (IIR), both of which were absent in astrocytoma cells. IIR constituted the predominant conductance in comparison astrocytes and was responsible for a high-resting K+ conductance in these cells. Voltage-activated Na+ currents were observed in 37 of 53 astrocytoma cells. Na+ current densities in astrocytoma cells, on average, were three- to fivefold larger than in comparison astrocytes. Astrocytoma cells expressing INa could be induced to generate slow action potential-like responses (spikes) by current injections. The threshold for generating such spikes was -34 mV (from a holding potential of -70 mV). The spike amplitude and time width were 52.5 mV and 12 ms, respectively. No spikes could be elicited in comparison astrocytes, although some of them expressed Na+ currents of similar size. Comparison of astrocytes to astrocytoma cells suggests that the apparent lack of IIR, which leads to high-input resistance (>500 MOmega), allows glioma cells to be sufficiently depolarized to generate Na+ spikes, whereas the high resting K+ conductance in astrocytes prevents their depolarization and thus generation of spikes. Consistent with this notion, Na+ spikes could be induced in spinal cord astrocytes in culture when IIR was experimentally blocked by 10 microM Ba2+, suggesting that the absence of IIR in astrocytoma cells is primarily responsible for the unusual spiking behavior seen in these glial tumor cells. It is unlikely that such glial spikes ever occur in vivo.

Evidence for a high free radical state in low-grade astrocytomas

OBJECTIVE

It is postulated that reactive oxygen species may play an inductive role in neuro-oncogenesis. However, data pertaining to the redox state of astrocytomas are limited, which prompted us to undertake this study.

METHODS

Intraoperative snap-frozen samples were obtained from the surface and core of 8 low-grade and 11 high-grade astrocytomas. Small portions of each specimen were fixed in 10% neutral formalin or cacodylate-buffered glutaraldehyde. Lipid peroxidation was estimated by measuring thiobarbituric acid-reactive substances, and total glutathione levels were determined. Light microscopy was performed to define the relevant histopathology, and electron microscopy was used to quantitate peroxisomal content.

RESULTS

Thiobarbituric acid-reactive substance values for low-grade astrocytomas were significantly elevated compared to those for malignant lesions, as was the case for total glutathione. This discrepancy was especially marked at the tumor surface. Peroxisomes predominated in the low-grade category.

CONCLUSION

We speculate regarding malignant transformation as a possible consequence of this decline in antioxidant capacity, as well as regarding the role of seizures and astrocytoma glutamate receptors in the initiation of free radical cascades. The therapeutic and teleological implications are considerable.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Properties of angiotensin II receptors in glial cells from the adult corpus callosum

The existence and the properties of angiotensin II receptors in the adult bovine and human corpus callosum (CC) were investigated by using Xenopus oocytes and primary glial cell cultures. In oocytes injected with CC mRNA, angiotensin II elicited oscillatory Cl- currents due to activation of the inositol phosphate/Ca(2+)-receptor-channel coupling system. The receptors expressed in oocytes and in CC cultures were pharmacologically similar to the AT1 receptor type as assayed by binding. Northern blot analysis and in situ hybridization studies in sections from CC and in glial cultures revealed that the receptors were molecularly related to the AT1 receptor and that they were present in astrocytes. In these cells, activation of the receptors with angiotensin II increased de novo DNA synthesis, promoted the release of aldosterone, and induced c-Fos expression. These findings indicate that CC astrocytes possess functional AT1 receptors that participate in various physiological processes.

mRNAs coding for neurotransmitter receptors in rabbit and rat visual areas

Levels of mRNAs encoding neurotransmitter receptors in the visual cortex, lateral geniculate nucleus, and superior colliculus of the rabbit and rat, and properties of the receptors expressed, were studied using Xenopus laevis oocytes. mRNA extracted from these areas was injected into the oocytes, which then acquired functional receptors. Electrical recordings of neurotransmitter-induced membrane currents reflect the relative amounts of mRNAs encoding the corresponding receptors. Receptors to gamma aminobutyric acid (GABA), kainate, glutamate, and serotonin exhibited uniformly high levels of expression, whereas expression of receptors to glycine and N-methyl-D-aspartate was uniformly low. In contrast, the expression of receptors to acetylcholine and substance P was highly non-uniform. Expression of acetylcholine receptors was high in oocytes injected with mRNA from the visual cortex, low for the lateral geniculate nucleus, and very low or absent for the superior colliculus. Conversely, the currents elicited by substance P were large in oocytes injected with superior colliculus mRNA, but were small or absent in oocytes injected with mRNAs from the other regions. Immunohistochemical analysis, at the light and electron microscopic levels, was used to localize choline acetyltransferase, the acetylcholine-synthesizing enzyme, and substance P-containing synaptic boutons in the three visual areas. Their presence closely paralleled the potency of mRNAs coding for acetylcholine and substance P receptors. The ability of rat mRNA, from each visual area, to induce neurotransmitter receptors was similar to that observed in the corresponding rabbit mRNAs. In addition to the marked differential distribution of mRNA encoding neurotransmitter receptors in the visual system, our findings reveal the probable existence of as yet uncharacterized receptors, whose new molecular forms may be revealed by further study. Our results also provide the basic information required for subsequent studies on the effect of monocular deprivation on the expression of neurotransmitter receptors in the visual system.

Neurotransmitter receptors and voltage-dependent Ca2+ channels encoded by mRNA from the adult corpus callosum

The presence of mRNAs encoding neurotransmitter receptors and voltage-gated channels in the adult human and bovine corpus callosum was investigated using Xenopus oocytes. Oocytes injected with mRNA extracted from the corpus callosum expressed functional receptors to glutamate, acetylcholine, and serotonin, and also voltage-operated Ca2+ channels, all with similar properties in the two species studied. Acetylcholine and serotonin elicited oscillatory Cl- currents due to activation of the inositol phosphate-Ca2+ receptor-channel coupling system. Glutamate and its analogs N-methyl-D-aspartate (NMDA), kainate, quisqualate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) induced smooth currents. The non-NMDA responses showed a strong inward rectification at positive potentials and were potently blocked by 6,7-dinitroquinoxaline-2,3-dione, as observed for the AMPA/kainate glutamate receptors GLUR1 and GLUR3. Furthermore, in situ hybridization experiments showed that GLUR1 and GLUR3 mRNAs are present in corpus callosum cells that were labeled with antiserum to glial fibrillary acid protein and that, in primary cell cultures, had the morphology of type 2 astrocytes. These results indicate that glial cells in the adult corpus callosum possess mRNA encoding functional neurotransmitter receptors and Ca2+ channels. These molecules may provide a mechanism for glial-neuronal interactions.