Mechanisms for L-channel-mediated increase in [Ca(2+)]i and its reduction by anti-bipolar drugs in cultured astrocytes combined with its mRNA expression in freshly isolated cells support the importance of astrocytic L-channels

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Astrocytes in rare neurological conditions: Morphological and functional considerations

Astrocytes are a population of central nervous system (CNS) cells with distinctive morphological and functional characteristics that differ within specific areas of the brain and are widely distributed throughout the CNS. There are mainly two types of astrocytes, protoplasmic and fibrous, which differ in morphologic appearance and location. Astrocytes are important cells of the CNS that not only provide structural support, but also modulate synaptic activity, regulate neuroinflammatory responses, maintain the blood-brain barrier, and supply energy to neurons. As a result, astrocytic disruption can lead to widespread detrimental effects and can contribute to the pathophysiology of several neurological conditions. The characteristics of astrocytes in more common neuropathologies such as Alzheimer's and Parkinson's disease have significantly been described and continue to be widely studied. However, there still exist numerous rare neurological conditions in which astrocytic involvement is unknown and needs to be explored. Accordingly, this review will summarize functional and morphological changes of astrocytes in various rare neurological conditions based on current knowledge thus far and highlight remaining neuropathologies where astrocytic involvement has yet to be investigated.

Ammonium induced dysfunction of 5-HT2B receptor in astrocytes

Previously we reported that gene expression of astrocytic 5-HT_2B receptors was decreased in brains of depressed animals exposed to chronic mild stress (CMS) (Li et al., 2012) and of Parkinson's disease (Song et al., 2018). Depression is also one of the psychiatric symptoms in hyperammonemia, and astrocyte is a primary target of ammonium in brain in vivo. In the present study, we have used preparations of the brains of urease-treated mice and ammonium-treated astrocytes in culture to study gene expression and function of 5-HT_2B receptors. The urease-treated mice showed depressive behaviour. Both mRNA and protein of 5-HT_2B receptors were increased in the brains of urease-treated mice and in ammonium-treated cultured astrocytes. Further study revealed that mRNA and protein expression of adenosine deaminase acting on RNA 2 (ADAR2), an enzyme catalyze RNA deamination of adenosine to inosine was increased in the brains of urease-treated mice and in ammonium-treated cultured astrocytes. This increase in ADAR2 induced RNA editing of 5-HT_2B receptors. Cultured astrocytes treated with ammonium lost 5-HT induced Ca²⁺ signalling and ERK_1/2 phosphorylation, indicating dysfunction of 5-HT_2B receptors. This is in agreement with our previous observation that edited 5-HT_2B receptors no longer respond to 5-HT (Hertz et al., 2014). Ammonium effects are inhibited by ADAR2 siRNA in cultured astrocytes, suggesting that increased gene expression and editing and loss of function of 5-HT_2B receptors are results of increased activity of ADAR2. In summary, we have demonstrated that functional malfunction of astrocytic 5-HT_2B receptors occurs in animal models of major depression, Parkinson depression and hepatic encephalopathy albeit via different mechanisms. Understanding the role of astrocytic 5-HT_2B receptors in different pathological contexts may instigate development of novel therapeutic strategies for treating disease-specific depressive behaviour.

Glycogenolysis in Acquired Glioma Resistance to Temozolomide: A Role for the [Ca2+]i-dependent Activation of Na,K-ATPase/ERK1/2 Signaling

Understanding the mechanistic basis for temozolomide (TMZ)-induced glioma resistance is an important obstacle in developing an effective form of chemotherapy for this type of tumor. Glycogenolysis is known to play an essential role in cellular proliferation and potassium homeostasis and involves the glycogen phosphorylase isoenzyme BB (GPBB). In this investigation, plasma GPBB was correlated with TMZ-resistance. Elevated plasma GPBB concentrations were found to be more frequent in a TMZ-resistant cohort of patients with poor survival rates. TMZ inhibits cell proliferation and induces TMZ resistance by upregulating the expression of O(6)-methylguanine-DNA methyltransferase (MGMT). This process requires glycogenolysis, which was confirmed herein by treatment with 1,4-dideoxy-1,4-imino-D-arabinitol hydrochloride, a glycogenolysis inhibitor and a special GPBB inhibitor. Acute TMZ treatment leads to upregulation of [Ca²⁺]_i, extracellular-regulated kinase (ERK)_1/2 phosphorylation, and chronic TMZ treatment leads to upregulation of the expression of Na,K-ATPase, ERK_1/2, and MGMT protein. Upregulation was abolished for each of these by inhibitors of transient receptor potential channel 1 and the inositol trisphosphate receptor. L-channel [Ca²⁺]_i inhibitors and RyR antagonists had no such effect. These results demonstrate that [Ca²⁺]_i-dependent glycogenolysis participates in acquired glioma TMZ-resistance by upregulating MGMT via a Na,K-ATPase/ERK_1/2 signaling pathway. GPBB and glycogenolysis may therefore represent novel therapeutic targets for overcoming TMZ-resistant gliomas.

Circadian ATP Release in Organotypic Cultures of the Rat Suprachiasmatic Nucleus Is Dependent on P2X7 and P2Y Receptors

The circadian rhythms in physiological and behavioral functions are driven by a pacemaker located in the suprachiasmatic nucleus (SCN). The rhythms continue in constant darkness and depend on cell-cell communication between neurons and glia. The SCN astrocytes generate also a circadian rhythm in extracellular adenosine 5′-triphosphate (ATP) accumulation, but molecular mechanisms that regulate ATP release are poorly understood. Here, we tested the hypothesis that ATP is released via the plasma membrane purinergic P2X7 receptors (P2X7Rs) and P2Y receptors (P2YRs) which have been previously shown to be expressed in the SCN tissue at transcriptional level. We have investigated this hypothesis using SCN organotypic cultures, primary cultures of SCN astrocytes, ATP bioluminescent assays, immunohistochemistry, patch-clamping, and calcium imaging. We found that extracellular ATP accumulation in organotypic cultures followed a circadian rhythm, with a peak between 24:00 and 04:00 h, and the trough at ~12:00 h. ATP rhythm was inhibited by application of AZ10606120, A438079, and BBG, specific blockers of P2X7R, and potentiated by GW791343, a positive allosteric modulator of this receptor. Double-immunohistochemical staining revealed high expression of the P2X7R protein in astrocytes of SCN slices. PPADS, a non-specific P2 antagonist, and MRS2179, specific P2Y1R antagonist, also abolished ATP rhythm, whereas the specific P2X4R blocker 5-BDBD was not effective. The pannexin-1 hemichannel blocker carbenoxolone displayed a partial inhibitory effect. The P2Y1R agonist MRS2365, and the P2Y2R agonist MRS2768 potentiated ATP release in organotypic cultures and increase intracellular Ca²⁺ level in cultured astrocytes. Thus, SCN utilizes multiple purinergic receptor systems and pannexin-1 hemichannels to release ATP.

Glycogenolysis, an Astrocyte-Specific Reaction, is Essential for Both Astrocytic and Neuronal Activities Involved in Learning

In brain glycogen, formed from glucose, is degraded (glycogenolysis) in astrocytes but not in neurons. Although most of the degradation follows the same pathway as glucose, its breakdown product, l-lactate, is released from astrocytes in larger amounts than glucose when glycogenolysis is activated by noradrenaline. However, this is not the case when glycogenolysis is activated by high potassium ion (K⁺) concentrations - possibly because noradrenaline in contrast to high K⁺ stimulates glycogenolysis by an increase not only in free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) but also in cyclic AMP (c-AMP), which may increase the expression of the monocarboxylate transporter through which it is released. Several transmitters activate glycogenolysis in astrocytes and do so at different time points after training. This stimulation is essential for memory consolidation because glycogenolysis is necessary for uptake of K⁺ and stimulates formation of glutamate from glucose, and therefore is needed both for removal of increased extracellular K⁺ following neuronal excitation (which initially occurs into astrocytes) and for formation of transmitter glutamate and GABA. In addition the released l-lactate has effects on neurons which are essential for learning and for learning-related long-term potentiation (LTP), including induction of the neuronal gene Arc/Arg3.1 and activation of gene cascades mediated by CREB and cofilin. Inhibition of glycogenolysis blocks learning, LTP and all related molecular events, but all changes can be reversed by injection of l-lactate. The effect of extracellular l-lactate is due to both astrocyte-mediated signaling which activates noradrenergic activity on all brain cells and to a minor uptake, possibly into dendritic spines.

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Chronic Treatment with Anti-bipolar Drugs Down-Regulates Gene Expression of TRPC1 in Neurones

In the brain, TRPC1 channels are abundantly expressed in neurones virtually in all regions; these proteins function as receptor-activated ion channels and are implicated in numerous processes, being specifically important for neurogenesis. Primary cultures of mouse cerebellar granule cell, cerebral cortical neurones, and freshly isolated neurones from in vivo brains were used to study effects of chronic treatment with anti-bipolar drugs [carbamazepine (CBZ), lithium salts and valproic acid] on gene expression of TRPC1. Expression of TRPC1 mRNA was identified with reverse transcription-polymerase chain reaction, whereas protein content was determined by Western blotting. Store-operated plasmalemmal Ca²⁺ entry (SOCE) was measured with fura-2 based microfluorimetry. Chronic treatment with each of the three drugs down-regulated mRNA and protein expression in cultured cerebellar granule cells in a time- and concentration-dependent manner. Similar effect was also observed in cultured cerebral cortical neurones treated with CBZ, lithium salts and valproic acid and in freshly isolated neurones from the brains of CBZ-treated animals. The amplitude of SOCE was substantially decreased in cerebellar granule cells chronically treated with each of the three drugs. Our findings indicate that down-regulation of TRPC1 gene expression and function in neurones may be one of the mechanisms of anti-bipolar drugs action.

Astrocytes regulate heterogeneity of presynaptic strengths in hippocampal networks

Dendrites are neuronal structures specialized for receiving and processing information through their many synaptic inputs. How input strengths are modified across dendrites in ways that are crucial for synaptic integration and plasticity remains unclear. We examined in single hippocampal neurons the mechanism of heterosynaptic interactions and the heterogeneity of synaptic strengths of pyramidal cell inputs. Heterosynaptic presynaptic plasticity that counterbalances input strengths requires N-methyl-d-aspartate receptors (NMDARs) and astrocytes. Importantly, this mechanism is shared with the mechanism for maintaining highly heterogeneous basal presynaptic strengths, which requires astrocyte Ca(2+) signaling involving NMDAR activation, astrocyte membrane depolarization, and L-type Ca(2+) channels. Intracellular infusion of NMDARs or Ca(2+)-channel blockers into astrocytes, conditionally ablating the GluN1 NMDAR subunit, or optogenetically hyperpolarizing astrocytes with archaerhodopsin promotes homogenization of convergent presynaptic inputs. Our findings support the presence of an astrocyte-dependent cellular mechanism that enhances the heterogeneity of presynaptic strengths of convergent connections, which may help boost the computational power of dendrites.

Targeting astrocytes in bipolar disorder

Astrocytes are homeostatic cells of the central nervous system, which are critical for development and maintenance of synaptic transmission and hence of synaptically connected neuronal ensembles. Astrocytic densities are reduced in bipolar disorder, and therefore deficient astroglial function may contribute to overall disbalance in neurotransmission and to pathological evolution. Classical anti-bipolar drugs (lithium salts, valproic acid and carbamazepine) affect expression of astroglial genes and modify astroglial signalling and homeostatic cascades. Many effects of both antidepressant and anti-bipolar drugs are exerted through regulation of glutamate homeostasis and glutamatergic transmission, through K(+) buffering, through regulation of calcium-dependent phospholipase A2 (that controls metabolism of arachidonic acid) or through Ca(2+) homeostatic and signalling pathways. Sometimes anti-depressant and anti-bipolar drugs exert opposite effects, and some effects on gene expression in drug treated animals are opposite in neurones vs. astrocytes. Changes in the intracellular pH induced by anti-bipolar drugs affect uptake of myo-inositol and thereby signalling via inositoltrisphosphate (InsP3), this being in accord with one of the main theories of mechanism of action for these drugs.

IL-17 induces MIP-1α expression in primary mouse astrocytes via TRPC channel

Our previous study demonstrated IL-17-mediated induction of MIP-1α through its binding to the cognate IL-17RA and MIP-1α was involved in astrocyte activation. Transient receptor potential canonical (TRPC) channel was involved in astrocyte activation, however, whether TRPC channel regulates MIP-1α expression in the context of multiple sclerosis (MS) remains largely unknown. In this study we identify the essential role of TRPC channel in IL-17-mediated MIP-1α expression and astrocyte activation. Moreover, treatment of astrocytes with IL-17 activated MAPKs and PI3K/Akt signaling pathways with downstream NF-κB pathways. Interestingly, the TRPC blocker-SKF96365 (10 μM) and Norgestimate (10 μM) significantly inhibited the increased expression of MIP-1α via suppression of IL-17-mediated ERK, p38 and JNK MAPKs and PI3K/Akt pathway activation, thereby underscoring the role of TRPC channel in this process. Together these data underpin the role of TRPC channel as a novel target that regulates MIP-1α expression and cell activation-mediated by IL-17 with implications for therapeutic intervention for reversal of neuroinflammation inflicted by IL-17. Understanding the regulation of MIP-1α expression may provide insights into the development of potential therapeutic targets for neuroinflammation associated with MS.

Antiepileptic drugs in patients with malignant brain tumor: beyond seizures and pharmacokinetics

In neurological malignancies, antiepileptic drugs (AEDs) are frequently used to control the seizure activity that accompanies the disorder. There is a growing body of evidence on the importance of AED selection for reasons other than pharmacokinetics (PK) properties. Epigenetic modifications may occur in glioblastomas, such as changes in gene methylation and histone acetylation states. Secondary mechanisms of AED drug action which impact these epigenetic modifications could play a significant role in patient survival outcomes. Both valproic acid (VPA) and carbamazepine have histone deacetylase (HDAC) inhibitory activities, and levetiracetam and VPA reduce the activity of O6-methylguanine-DNA methyltransferase (MGMT), a DNA-repair molecule implicated in resistance to alkylating agents used for chemotherapy. The use of AEDs for purposes other than seizure prophylaxis and their selection based on non-PK properties present a potential paradigm shift in the field of neuro-oncology.

Calcium channel genes associated with bipolar disorder modulate lithium's amplification of circadian rhythms

Bipolar disorder (BD) is associated with mood episodes and low amplitude circadian rhythms. Previously, we demonstrated that fibroblasts grown from BD patients show weaker amplification of circadian rhythms by lithium compared to control cells. Since calcium signals impact upon the circadian clock, and L-type calcium channels (LTCC) have emerged as genetic risk factors for BD, we examined whether loss of function in LTCCs accounts for the attenuated response to lithium in BD cells. We used fluorescent dyes to measure Ca(2+) changes in BD and control fibroblasts after lithium treatment, and bioluminescent reporters to measure Per2::luc rhythms in fibroblasts from BD patients, human controls, and mice while pharmacologically or genetically manipulating calcium channels. Longitudinal expression of LTCC genes (CACNA1C, CACNA1D and CACNB3) was then measured over 12-24 h in BD and control cells. Our results indicate that independently of LTCCs, lithium stimulated intracellular Ca(2+) less effectively in BD vs. control fibroblasts. In longitudinal studies, pharmacological inhibition of LTCCs or knockdown of CACNA1A, CACNA1C, CACNA1D and CACNB3 altered circadian rhythm amplitude. Diltiazem and knockdown of CACNA1C or CACNA1D eliminated lithium's ability to amplify rhythms. Knockdown of CACNA1A or CACNB3 altered baseline rhythms, but did not affect rhythm amplification by lithium. In human fibroblasts, CACNA1C genotype predicted the amplitude response to lithium, and the expression profiles of CACNA1C, CACNA1D and CACNB3 were altered in BD vs.

CONTROLS

We conclude that in cells from BD patients, calcium signaling is abnormal, and that LTCCs underlie the failure of lithium to amplify circadian rhythms.

Levetiracetam, Calcium Antagonism, and Bipolar Disorder

Hyperactive intracellular calcium ion (Ca) signaling in peripheral cells has been a reliable finding in bipolar disorder. Some established mood stabilizing medications, such as lithium and carbamazepine, have been found to normalize elevated intracellular Ca concentrations ([Ca]i) in platelets and lymphocytes from bipolar disorder patients, and some medications the primary effect of which is to attenuate increased [Ca]i have been reported to have mood stabilizing properties.Hyperactive intracellular Ca signaling has also been implicated in epilepsy, and some anticonvulsants have calcium antagonist properties. This study demonstrated that levetiracetam, an anticonvulsant that has been shown to block N and P/Q-type calcium channels in animal studies does not alter elevated [Ca]i in blood platelets of patients with bipolar disorder. Review of published clinical trials revealed no controlled evidence of efficacy as a mood stabilizer.This study underscores the possibility that pharmacologic actions of a medication in animals and normal subjects may not necessarily predict its pharmacologic or clinical effects in actual patients. Effects of treatments on pathophysiology that is demonstrated in clinical subtypes may be more likely to predict effectiveness in those subtypes than choosing medications based on structural similarities to established treatments.

Ammonium increases Ca(2+) signalling and upregulates expression of Cav1.2 gene in astrocytes in primary cultures and in the in vivo brain

AIM

The primary aim of this study was to identify the effects of hyperammonaemia on functional expression of Cav1.2 L-type Ca(2+) channels in astroglia.

METHODS

Primary cultures of mouse astrocytes were used to study effects of chronic treatment (1-5 days) with ammonium chloride, at 1, 3 and 5 mm on depolarization-induced increases in free cytosolic Ca(2+) concentration ([Ca(2+)]i , measured with Fura-2 based microfluorimetry) in control conditions and following treatment with the L-type Ca(2+) channel inhibitor, nifedipine, or with ryanodine receptor inhibitor, ryanodine. Expression of Cav1.2 mRNA was identified with RT-PCR, whereas protein content was determined by Western blotting. Sustained hyperammonaemia in vivo was induced by daily injections of urease (33 units kg body weight(-1), i.p.) for 3 days.

RESULTS

Depolarization-induced [Ca(2+)]i transients sensitive to nifedipine (peak of the response) and to ryanodine (plateau phase) were significantly increased in astrocytes chronically exposed to ammonium. The ammonium-induced increase in Ca(2+) influx in astrocytes resulted from an upregulation of Cav1.2 channel's expression detected at mRNA and protein levels. Increase in Cav1.2 expression was prevented by ouabain antagonist canrenone. Similar upregulation of Cav1.2 gene expression was found in the brains of adult mice subjected to intraperitoneal injection of urease. In transgenic mice tagged with an astrocyte-specific or neurone-specific markers and treated with intraperitoneal injections of urease, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that Cav1.2 mRNA expression was upregulated in astrocytes, but not in neurones.

CONCLUSIONS

Ammonium-induced deregulation of astroglial Ca(2+) signalling, is, in part, associated with upregulation of Cav1.2 L-type calcium channels.

Chronic SSRI stimulation of astrocytic 5-HT2B receptors change multiple gene expressions/editings and metabolism of glutamate, glucose and glycogen: a potential paradigm shift

It is firmly believed that the mechanism of action of SSRIs in major depression is to inhibit the serotonin transporter, SERT, and increase extracellular concentration of serotonin. However, this undisputed observation does not prove that SERT inhibition is the mechanism, let alone the only mechanism, by which SSRI's exert their therapeutic effects. It has recently been demonstrated that 5-HT2B receptor stimulation is needed for the antidepressant effect of fluoxetine in vivo. The ability of all five currently used SSRIs to stimulate the 5-HT2B receptor equipotentially in cultured astrocytes has been known for several years, and increasing evidence has shown the importance of astrocytes and astrocyte-neuronal interactions for neuroplasticity and complex brain activity. This paper reviews acute and chronic effects of 5-HT2B receptor stimulation in cultured astrocytes and in astrocytes freshly isolated from brains of mice treated with fluoxetine for 14 days together with effects of anti-depressant therapy on turnover of glutamate and GABA and metabolism of glucose and glycogen. It is suggested that these events are causally related to the mechanism of action of SSRIs and of interest for development of newer antidepressant drugs.

Chronic treatment with anti-bipolar drugs suppresses glutamate release from astroglial cultures

Astroglial cells are fundamental elements of most neurological diseases, including bipolar disorders in which astrocytes show morphological and functional deficiency. Here we report the suppression of astroglial glutamate release by chronic treatment with three anti-bipolar drugs, lithium salt (Li(+)), carbamazepine (CBZ) and valproic acid (VPA). Release of glutamate was triggered by transient exposure of astrocytes to ATP (which activated purinoceptors) and 45 mM K(+) (which depolarised cell membrane to ~-30 mV). In both types of stimulation glutamate release was regulated by Ca(2+) entry through plasmalemmal channels and by Ca(2+) release from the endoplasmic reticulum (ER) intracellular stores. Exposure of astroglial cultures to Li(+), CBZ and VPA for 2 weeks led to a significant (more than 2 times) inhibition of glutamate release, which may alleviate the hyperactivity of the glutamatergic transmission in the brain of patients with bipolar disorders and thus contribute the underlying mechanism of drug action.

Astrocytic glycogenolysis: mechanisms and functions

Until the demonstration little more than 20 years ago that glycogenolysis occurs during normal whisker stimulation glycogenolysis was regarded as a relatively uninteresting emergency procedure. Since then, a series of important astrocytic functions has been shown to be critically dependent on glycogenolytic activity to support the signaling mechanisms necessary for these functions to operate. This applies to glutamate formation and uptake and to release of ATP as a transmitter, stimulated by other transmitters or elevated K(+) concentrations and affecting not only other astrocytes but also most other brain cells. It is also relevant for astrocytic K(+) uptake both during the period when the extracellular K(+) concentration is still elevated after neuronal excitation, and capable of stimulating glycogenolytic activity, and during the subsequent undershoot after intense neuronal activity, when glycogenolysis may be stimulated by noradrenaline. Both elevated K(+) concentrations and several transmitters, including the β-adrenergic agonist isoproterenol and vasopressin increase free cytosolic Ca(2+) concentration in astrocytes, which stimulates phosphorylase kinase so that it activates the transformation of the inactive glycogen phosphorylase a to the active phosphorylase b. Contrary to common belief cyclic AMP plays at most a facilitatory role, and only when free cytosolic Ca(2+) concentration is also increased. Cyclic AMP is not increased during activation of glycogenolysis by either elevated K(+) concentrations or the stimulation of the serotonergic 5-HT(2B) receptor. Not all agents that stimulate glycogenolysis do so by directly activating phophorylase kinase--some do so by activating processes requiring glycogenolysis, e.g. for synthesis of glutamate.

Differential subcellular Ca2+ signaling in a highly specialized subpopulation of astrocytes

Recent evidence suggests that astrocytes do not serve a mere buffering function, but exhibit complex signaling pathways, disturbance of which contributes significantly to the pathophysiology of CNS diseases. Little is known regarding the intracellular signaling pathways in the specialized optic nerve head astrocytes (ONHAs), the major glia cell type in non-myelinated optic nerve head. Here we show the differential subcellular expression of intracellular Ca(2+) channels in ONHAs. Expression of type 1 and type 3 inositol-1-4-5,-trisphosphate receptors (IP3Rs) in the endoplasmic reticulum and type 2 IP3Rs in the nuclear envelope causes differential Ca(2+) release from intracellular stores in nuclear vs. cytosolic compartments. Our study identifies differential distribution and activity of Ca(2+) channels as molecular substrate and mechanism by which astrocytes independently regulate Ca(2+) transients in both cytoplasm and nucleoplasm, thereby controlling genomic and non-genomic cellular signaling, respectively. This provides excellent targets for therapeutics restoring pathological disturbances of intracellular Ca(2+) signaling present in glaucoma and other neurodegenerative disorders with astrocyte involvement.

Antagonists of the Vasopressin V1 Receptor and of the β(1)-Adrenoceptor Inhibit Cytotoxic Brain Edema in Stroke by Effects on Astrocytes - but the Mechanisms Differ

Brain edema is a serious complication in ischemic stroke because even relatively small changes in brain volume can compromise cerebral blood flow or result in compression of vital brain structures on account of the fixed volume of the rigid skull. Literature data indicate that administration of either antagonists of the V1 vasopressin (AVP) receptor or the β1-adrenergic receptor are able to reduce edema or infarct size when administered after the onset of ischemia, a key advantage for possible clinical use. The present review discusses possible mechanisms, focusing on the role of NKCC1, an astrocytic cotransporter of Na(+), K(+), 2Cl(-) and water and its activation by highly increased extracellular K(+) concentrations in the development of cytotoxic cell swelling. However, it also mentions that due to a 3/2 ratio between Na(+) release and K(+) uptake by the Na(+),K(+)-ATPase driving NKCC1 brain extracellular fluid can become hypertonic, which may facilitate water entry across the blood-brain barrier, essential for development of edema. It shows that brain edema does not develop until during reperfusion, which can be explained by lack of metabolic energy during ischemia. V1 antagonists are likely to protect against cytotoxic edema formation by inhibiting AVP enhancement of NKCC1-mediated uptake of ions and water, whereas β1-adrenergic antagonists prevent edema formation because β1-adrenergic stimulation alone is responsible for stimulation of the Na(+),K(+)-ATPase driving NKCC1, first and foremost due to decrease in extracellular Ca(2+) concentration. Inhibition of NKCC1 also has adverse effects, e.g. on memory and the treatment should probably be of shortest possible duration.

Fluoxetine and all other SSRIs are 5-HT2B Agonists - Importance for their Therapeutic Effects

Fluoxetine and other serotonin-specific re-uptake inhibitors (SSRIs) are generally thought to owe their therapeutic potency to inhibition of the serotonin transporter (SERT). However, research in our laboratory showed that it affects, with relatively high affinity the 5-HT2B receptor in cultured astrocytes; this finding was confirmed by independent observations showing that fluoxetine loses its ability to elicit SSRI-like responses in behavioral assays in mice in which the 5-HT2B receptor was knocked-out genetically or inhibited pharmacologically. All clinically used SSRIs are approximately equipotent towards 5-HT2B receptors and exert their effect on cultured astrocytes at concentrations similar to those used clinically, a substantial difference from their effect on SERT. We have demonstrated up-regulation and editing of astrocytic genes for ADAR2, the kainate receptor GluK2, cPLA2 and the 5-HT2B receptor itself after chronic treatment of cultures, which do not express SERT and after treatment of mice (expressing SERT) for 2 weeks with fluoxetine, followed by isolation of astrocytic and neuronal cell fractionation. Affected genes were identical in both experimental paradigms. Fluoxetine treatment also altered Ca(2+) homeostatic cascades, in a specific way that differs from that seen after treatment with the anti-bipolar drugs carbamazepine, lithium, or valproic acid. All changes occurred after a lag period similar to what is seen for fluoxetine's clinical effects, and some of the genes were altered in the opposite direction by mild chronic inescapable stress, known to cause anhedonia, a component of major depression. In the anhedonic mice these changes were reversed by treatment with SSRIs.

Regulatory volume increase in astrocytes exposed to hypertonic medium requires β1 -adrenergic Na(+) /K(+) -ATPase stimulation and glycogenolysis

The cotransporter of Na(+) , K(+) , 2Cl(-) , and water, NKKC1, is activated under two conditions in the brain, exposure to highly elevated extracellular K(+) concentrations, causing astrocytic swelling, and regulatory volume increase in cells shrunk in response to exposure to hypertonic medium. NKCC1-mediated transport occurs as secondary active transport driven by Na(+) /K(+) -ATPase activity, which establishes a favorable ratio for NKCC1 operation between extracellular and intracellular products of the concentrations of Na(+) , K(+) , and Cl(-) × Cl(-) . In the adult brain, astrocytes are the main target for NKCC1 stimulation, and their Na(+) /K(+) -ATPase activity is stimulated by elevated K(+) or the β-adrenergic agonist isoproterenol. Extracellular K(+) concentration is normal during regulatory volume increase, so this study investigated whether the volume increase occurred faster in the presence of isoproterenol. Measurement of cell volume via live cell microscopic imaging fluorescence to record fluorescence intensity of calcein showed that this was the case at isoproterenol concentrations of ≥1 µM in well-differentiated mouse astrocyte cultures incubated in isotonic medium with 100 mM sucrose added. This stimulation was abolished by the β1 -adrenergic antagonist betaxolol, but not by ICI118551, a β2 -adrenergic antagonist. A large part of the β1 -adrenergic signaling pathway in astrocytes is known. Inhibitors of this pathway as well as the glycogenolysis inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol hydrochloride and the NKCC1 inhibitors bumetanide and furosemide abolished stimulation by isoproterenol, and it was weakened by the Na(+) /K(+) -ATPase inhibitor ouabain. These observations are of physiological relevance because extracellular hypertonicity occurs during intense neuronal activity. This might trigger a regulatory volume increase, associated with the post-excitatory undershoot.

Ammonium increases Ca(2+) signalling and up-regulates expression of TRPC1 gene in astrocytes in primary cultures and in the in vivo brain

Rapid rise in ammonium concentration in the brain is the major pathogenic factor in hepatic encephalopathy that is manifested by state of confusion, forgetfulness and irritability, psychotic symptoms, delusions, lethargy, somnolence and, in the terminal stages, coma. Primary cultures of mouse astrocytes were used to investigate effects of chronic treatment (3 days) with ammonium chloride (ammonium) at 3 mM, this being a relevant concentration for hepatic encephalopathy condition, on metabotropic receptor agonist-induced increases in free cytosolic Ca(2+) concentration [(Ca(2+))i], measured with fura-2 based microfluorimetry and on store-operated Ca(2+) entry (SOCE) activated following treatment with the SERCA inhibitor thapsigargin. The agonists used were the β-adrenergic agonist isoproterenol, the α2-adrenergic agonist dexmedetomidine, the InsP3 receptor (InsP3R) agonist adenophostin A and ryanodine receptor agonist 4-Chloro-m-cresol (4-CMC). Agonist-induced [Ca(2+)]i responses were significantly increased in astrocytes chronically exposed to ammonium. Similarly, the SOCE, meditated by the transient receptor potential channel 1 (TRPC1), was significantly augmented. The ammonium-induced increase in SOCE was a result of an up-regulation of mRNA and protein expression of TRPC1 in astrocytes. Increase in TRPC1 expression and in SOCE were both prevented by ouabain antagonist canrenone. Similar up-regulation of TRPC1 gene expression was found in the brain of adult mice subjected to intraperitoneal injection of urease for 3 days. In transgenic mice tagged with an astrocyte-specific or a neurone-specific markers and treated with intraperitoneal injections of urease for 3 days, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that TRPC1 mRNA expression was up-regulated in astrocytes, but not in neurones.

Ammonium activates ouabain-activated signalling pathway in astrocytes: therapeutic potential of ouabain antagonist

The causal role of ammonium in hepatic encephalopathy was identified in 1930s. Astroglial cells are primary cellular elements of hepatic encephalopathy which conceptually, can be considered a toxic astrogliopathology. Previously we have reported that acute exposure to ammonium activated ouabain/Na,K-ATPase signalling pathway, which includes Src, EGF receptor, Raf, Ras, MEK and ERK1/2. Chronic incubation of astrocytes with ammonium increased production of endogenous ouabain-like compound. Ouabain antagonist canrenone abolished effects of ammonium on astrocytic swelling, ROS production, and upregulation of gene expression and function of TRPC1 and Cav1.2. However, ammonium induces multiple pathological modifications in astrocytes, and some of them may be not related to this signalling pathway. In this review, we focus on the effect of ammonium on ouabain/Na,K-ATPase signalling pathway and its involvement in ammonium-induced ROS production, cell swelling and aberration of Ca(2+) signals in astrocytes. We also briefly discuss Na,K-ATPase, EGF receptor, endogenous ouabain and ouabain antagonist.

Signal Transduction in Astrocytes during Chronic or Acute Treatment with Drugs (SSRIs, Antibipolar Drugs, GABA-ergic Drugs, and Benzodiazepines) Ameliorating Mood Disorders

Chronic treatment with fluoxetine or other so-called serotonin-specific reuptake inhibitor antidepressants (SSRIs) or with a lithium salt “lithium”, carbamazepine, or valproic acid, the three classical antibipolar drugs, exerts a multitude of effects on astrocytes, which in turn modulate astrocyte-neuronal interactions and brain function. In the case of the SSRIs, they are to a large extent due to 5-HT_2B-mediated upregulation and editing of genes. These alterations induce alteration in effects of cPLA₂, GluK2, and the 5-HT_2B receptor, probably including increases in both glucose metabolism and glycogen turnover, which in combination have therapeutic effect on major depression. The ability of increased levels of extracellular K⁺ to increase [Ca²⁺]_i is increased as a sign of increased K⁺-induced excitability in astrocytes. Acute anxiolytic drug treatment with benzodiazepines or GABA_A receptor stimulation has similar glycogenolysis-enhancing effects. The antibipolar drugs induce intracellular alkalinization in astrocytes with lithium acting on one acid extruder and carbamazepine and valproic acid on a different acid extruder. They inhibit K⁺-induced and transmitter-induced increase of astrocytic [Ca²⁺]_i and thereby probably excitability. In several cases, they exert different changes in gene expression than SSRIs, determined both in cultured astrocytes and in freshly isolated astrocytes from drug-treated animals.

Role of glycogenolysis in stimulation of ATP release from cultured mouse astrocytes by transmitters and high K+ concentrations

This study investigates the role of glycogenolysis in stimulated release of ATP as a transmitter from astrocytes. Within the last 20 years our understanding of brain glycogenolysis has changed from it being a relatively uninteresting process to being a driving force for essential brain functions like production of transmitter glutamate and homoeostasis of potassium ions (K⁺) after their release from excited neurons. Simultaneously, the importance of astrocytic handling of adenosine, its phosphorylation to ATP and release of some astrocytic ATP, located in vesicles, as an important transmitter has also become to be realized. Among the procedures stimulating Ca²⁺-dependent release of vesicular ATP are exposure to such transmitters as glutamate and adenosine, which raise intra-astrocytic Ca²⁺ concentration, or increase of extracellular K⁺ to a depolarizing level that opens astrocytic L-channels for Ca²⁺ and thereby also increase intra-astrocytic Ca²⁺ concentration, a prerequisite for glycogenolysis. The present study has confirmed and quantitated stimulated ATP release from well differentiated astrocyte cultures by glutamate, adenosine or elevated extracellular K⁺ concentrations, measured by a luciferin/luciferase reaction. It has also shown that this release is virtually abolished by an inhibitor of glycogenolysis as well as by inhibitors of transmitter-mediated signaling or of L-channel opening by elevated K⁺ concentrations.

Stimulation of Ca²⁺-dependent astrocytic ATP release by glutamate, adenosine or high K⁺ concentration is abolished by inhibition of glycogenolysis, probably due to signaling inhibition. This is consistent with the K⁺-mediated pathway, but glycogenolysis-sensitive steps in the transmitter pathways are unknown.

Short- and long-term treatment of mouse cortical primary astrocytes with β-amyloid differentially regulates the mRNA expression of L-type calcium channels

BACKGROUND

It is well established that reactive astrocytes express L-type calcium channels (LTCC), but their functional role is completely unknown. We have recently shown that reactive astrocytes highly express the CaV1.2 α1-subunit around β-amyloid (Aβ) plaques in an Alzheimer mouse model. The aim of the present study was to explore whether Aβ peptides may regulate the mRNA expression of all LTCC subunits in primary mouse astrocytes in culture.

METHODS

Confluent primary astrocytes were incubated with 10 µg/ml of human or murine Aβ or the toxic fragment Aβ25-35 for 3 days or for 3 weeks. The LTCC subunits were determined by quantitative RT-PCR.

RESULTS

Our data show that murine Aβ42 slightly but significantly increased CaV1.2 and CaV1.3 expression when incubated for 3 days. This acute treatment with murine Aβ enhanced β2 and β3 mRNA levels but decreased α2δ-2 mRNA expression. When astrocytes were incubated for 3 weeks, the levels of CaV1.2 α1 were significantly decreased by the murine Aβ and the toxic fragment. As a control, the protein kinase C-ε activator DCP-LA displayed a decrease in CaV2.1 expression.

CONCLUSION

In conclusion, our data show that Aβ can differentially regulate LTCC expression in primary mouse astrocytes depending on incubation time.

Glycogenolysis and purinergic signaling

Both ATP and glutamate are on one hand essential metabolites in brain and on the other serve a signaling function as transmitters. However, there is the major difference that the flux in the pathway producing transmitter glutamate is comparable to the rate of glucose metabolism in brain, whereas that producing transmitter ATP is orders of magnitude smaller than the metabolic turnover between ATP and ADP. Moreover, de novo glutamate production occurs exclusively in astrocytes, whereas transmitter ATP is produced both in neurons and astrocytes. This chapter deals only with ATP and exclusively with its formation and release in astrocytes, and it focuses on potential associations with glycogenolysis, which is known to be indispensable for the synthesis of glutamate. Glycogenolysis is dependent upon an increase in free intracellular Ca(2+) concentration (Ca(2+)]i). It can be further stimulated by cAMP, but in contrast to widespread beliefs, cAMP can on its own not induce glycogenolysis. Astrocytes generate ATP from accumulated adenosine, and this process does not seem to require glycogenolysis. A minor amount of the generated ATP is utilized as a transmitter, and its synthesis requires the presence of the mainly intracellular nucleoside transporter ENT3. Many transmitters as well as extracellular K(+) concentrations high enough to open the voltage-sensitive L-channels for Ca(2+) cause a release of transmitter ATP from astrocytes. Adenosine and ATP induce release of ATP by action at several different purinergic receptors. The release evoked by transmitters or elevated K(+) concentrations is abolished by DAB, an inhibitor of glycogenolysis.

Astrocytic and neuronal accumulation of elevated extracellular K(+) with a 2/3 K(+)/Na(+) flux ratio-consequences for energy metabolism, osmolarity and higher brain function

Brain excitation increases neuronal Na⁺ concentration by 2 major mechanisms: (i) Na⁺ influx caused by glutamatergic synaptic activity; and (ii) action-potential-mediated depolarization by Na⁺ influx followed by repolarizating K⁺ efflux, increasing extracellular K⁺ concentration. This review deals mainly with the latter and it concludes that clearance of extracellular K⁺ is initially mainly effectuated by Na⁺,K⁺-ATPase-mediated K⁺ uptake into astrocytes, at K⁺ concentrations above ~10 mM aided by uptake of Na⁺,K⁺ and 2 Cl⁻ by the cotransporter NKCC1. Since operation of the astrocytic Na⁺,K⁺-ATPase requires K⁺-dependent glycogenolysis for stimulation of the intracellular ATPase site, it ceases after normalization of extracellular K⁺ concentration. This allows K⁺ release via the inward rectifying K⁺ channel K_ir4.1, perhaps after trans-astrocytic connexin- and/or pannexin-mediated K⁺ transfer, which would be a key candidate for determination by synchronization-based computational analysis and may have signaling effects. Spatially dispersed K⁺ release would have little effect on extracellular K⁺ concentration and allow K⁺ accumulation by the less powerful neuronal Na⁺,K⁺-ATPase, which is not stimulated by increases in extracellular K⁺. Since the Na⁺,K⁺-ATPase exchanges 3 Na⁺ with 2 K⁺, it creates extracellular hypertonicity and cell shrinkage. Hypertonicity stimulates NKCC1, which, aided by β-adrenergic stimulation of the Na⁺,K⁺-ATPase, causes regulatory volume increase, furosemide-inhibited undershoot in [K⁺]_e and perhaps facilitation of the termination of slow neuronal hyperpolarization (sAHP), with behavioral consequences. The ion transport processes involved minimize ionic disequilibria caused by the asymmetric Na⁺,K⁺-ATPase fluxes.