Chronic Treatment with Anti-bipolar Drugs Causes Intracellular Alkalinization in Astrocytes, Altering Their Functions

The Double-Edged Effect of Connexins and Pannexins of Glial Cells in Central and Peripheral Nervous System After Nerve Injury

Glial cells play pivotal roles in homeostatic regulation and driving reactive pathologic changes after nerve injury. Connexins (Cxs) and pannexins (Panxs) have emerged as seminal proteins implicated in cell-cell communication, exerting a profound impact on the response processes of glial cell activation, proliferation, protein synthesis and secretion, as well as apoptosis following nerve injury. These influences are mediated through various forms, including protein monomers, hemichannel (HC), and gap junction (GJ), mainly by regulating intercellular or intracellular signaling pathways. Multiple Cx and Panx isoforms have been detected in central nervous system (CNS) or peripheral nervous system (PNS). Each isoform exhibits distinct cellular and subcellular localization, and the differential regulation and functional roles of various protein isoforms are observed post-injury. The quantitative and functional alterations of the same protein isoform in different studies remain inconsistent, attributable to factors such as the predominant mode of protein polymerization, the specific injury model, and the injury site. Similarly, the same protein isoforms have different roles in regulating the response processes after nerve injury, thus exerting a double-edged sword effect. This review describes the regulatory mechanisms and bidirectional effects of Cxs and Panxs. Additionally, it surveys the current status of research and application of drugs as therapeutic targets for neuropathic injuries. We summarize comprehensive and up-to-date information on these proteins in the glial cell response to nerve injury, providing new perspectives for future mechanistic exploration and development of targeted therapeutic approaches.

Potential for New Therapeutic Approaches by Targeting Lactate and pH Mediated Epigenetic Dysregulation in Major Mental Diseases

Multiple lines of evidence have shown that lactate-mediated pH alterations in the brains of patients with neuropsychiatric diseases such as schizophrenia (SCZ), Alzheimer's disease (AD) and autism may be attributed to mitochondrial dysfunction and changes in energy metabolism. While neuronal activity is associated with reduction in brain pH, astrocytes are responsible for rebalancing the pH to maintain the equilibrium. As lactate level is the main determinant of brain pH, neuronal activities are impacted by pH changes due to the binding of protons (H+) to various types of proteins, altering their structure and function in the neuronal and non-neuronal cells of the brain. Lactate and pH could affect diverse types of epigenetic modifications, including histone lactylation, which is linked to histone acetylation and DNA methylation. In this review, we discuss the importance of pH homeostasis in normal brain function, the role of lactate as an essential epigenetic regulatory molecule and its contributions to brain pH abnormalities in neuropsychiatric diseases, and shed light on lactate-based and pH-modulating therapies in neuropsychiatric diseases by targeting epigenetic modifications. In conclusion, we attempt to highlight the potentials and challenges of translating lactate-pH-modulating therapies to clinics for the treatment of neuropsychiatric diseases.

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.

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.

Astrocyte Cultures Mimicking Brain Astrocytes in Gene Expression, Signaling, Metabolism and K+ Uptake and Showing Astrocytic Gene Expression Overlooked by Immunohistochemistry and In Situ Hybridization

Based on differences in gene expression between cultured astrocytes and freshly isolated brain astrocytes it has been claimed that cultured astrocytes poorly reflect the characteristics of their in vivo counterparts. This paper shows that this is not the case with the cultures of mouse astrocytes we have used since 1978. The culture is prepared following guidelines provided by Drs. Monique Sensenbrenner and John Booher, with the difference that dibutyryl cyclic AMP is added to the culture medium from the beginning of the third week. This addition has only minor effects on glucose and glutamate metabolism, but it is crucial for effects by elevated K⁺ concentrations and for Ca²⁺ homeostasis, important aspects of astrocyte function. Work by Liang Peng and her colleagues has shown identity between not only gene expression but also drug-induced gene upregulations and editings in astrocytes cultured by this method and astrocytes freshly isolated from brains of drug-treated animals. Dr. Norenberg's laboratory has demonstrated identical upregulation of the cotransporter NKCC1 in ammonia-exposed astrocytes and rats with liver failure. Similarity between cultured and freshly isolated astrocytes has also been shown in metabolism, K⁺ uptake and several aspects of signaling. However, others have shown that the gene for the glutamate transporter GLT1 is not expressed, and rat cultures show some abnormalities in K⁺ effects. Nevertheless, the overall reliability of the cultured cells is important because immunohistochemistry and in situ hybridization poorly demonstrate many astrocytic genes, e.g., those of nucleoside transporters, and even microarray analysis of isolated cells can be misleading.

Glial pathology in bipolar disorder: potential therapeutic implications

Bipolar disorder (BD) is a chronic and severe mental disorder with recurrent episodes of mania and depression. In addition to neuronal alterations, accumulating evidences have revealed the importance of glial system in pathophysiology and phenotype of the illness. Postmortem studies have repeatedly demonstrated the alterations in glial cells and its functions in patients with BD. The activated microglia and inflammatory cytokines are proposed to be the potential biomarkers that may help to predict disease exacerbation in BD. On the other hand, anti-BD drugs have been shown to produce profound effects on glial activity, which not only contributes to the therapeutic efficacy, but may also provide a potential target for the drug development of BD. We will focus on the recent development of glial abnormalities and potential therapeutic benefits targeted to glial modulation in BD.

Fluoxetine induces alkalinization of astroglial cytosol through stimulation of sodium-hydrogen exchanger 1: dissection of intracellular signaling pathways

Clinical evidence suggest astrocytic abnormality in major depression (MD) while treatment with anti-psychotic drugs affects astroglial functions. Astroglial cells are involved in pH homeostasis of the brain by transporting protons (through sodium-proton transporter 1, NHE1, glutamate transporters EAAT1/2 and proton-lactate co-transporter MCT1) and bicarbonate (through the sodium-bicarbonate co-transporter NBC or the chloride-bicarbonate exchanger AE). Here we show that chronic treatment with fluoxetine increases astroglial pH_i by stimulating NHE1-mediated proton extrusion. At a clinically relevant concentration of 1 μM, fluoxetine significantly increased astroglial pH_i from 7.05 to 7.34 after 3 weeks and from 7.18 to 7.58 after 4 weeks of drug treatment. Stimulation of NHE1 is a result of transporter phosphorylation mediated by several intracellular signaling cascades that include MAPK/ERK_1/2, PI3K/AKT and ribosomal S6 kinase (RSK). Fluoxetine stimulated phosphorylation of ERK_1/2, AKT and RSK in a concentration dependent manner. Positive crosstalk exists between two signal pathways, MAPK/ERK_1/2 and PI3K/AKT activated by fluoxetine since ERK_1/2 phosphrylation could be abolished by inhibitors of PI3K, LY294002 and AKT, triciribine, and AKT phosphorylation by inhibitor of MAPK, U0126. As a result, RSK phosphorylation was not only inhibited by U0126 but also by inhibitor of LY294002. The NHE1 phoshorylation resulted in stimulation of NHE1 activity as revealed by the NH₄Cl-prepulse technique; the increase of NHE1 activity was dependent on fluoxetine concentration, and could be inhibited by both U0126 and LY294002. Our findings suggest that regulation of astrocytic pH_i and brain pH may be one of the mechanisms underlying fluoxetine action.

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.

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.

Methodological limitations in determining astrocytic gene expression

Traditionally, astrocytic mRNA and protein expression are studied by in situ hybridization (ISH) and immunohistochemically. This led to the concept that astrocytes lack aralar, a component of the malate-aspartate-shuttle. At least similar aralar mRNA and protein expression in astrocytes and neurons isolated by fluorescence-assisted cell sorting (FACS) reversed this opinion. Demonstration of expression of other astrocytic genes may also be erroneous. Literature data based on morphological methods were therefore compared with mRNA expression in cells obtained by recently developed methods for determination of cell-specific gene expression. All Na,K-ATPase-α subunits were demonstrated by immunohistochemistry (IHC), but there are problems with the cotransporter NKCC1. Glutamate and GABA transporter gene expression was well determined immunohistochemically. The same applies to expression of many genes of glucose metabolism, whereas a single study based on findings in bacterial artificial chromosome (BAC) transgenic animals showed very low astrocytic expression of hexokinase. Gene expression of the equilibrative nucleoside transporters ENT1 and ENT2 was recognized by ISH, but ENT3 was not. The same applies to the concentrative transporters CNT2 and CNT3. All were clearly expressed in FACS-isolated cells, followed by biochemical analysis. ENT3 was enriched in astrocytes. Expression of many nucleoside transporter genes were shown by microarray analysis, whereas other important genes were not. Results in cultured astrocytes resembled those obtained by FACS. These findings call for reappraisal of cellular nucleoside transporter expression. FACS cell yield is small. Further development of cell separation methods to render methods more easily available and less animal and cost consuming and parallel studies of astrocytic mRNA and protein expression by ISH/IHC and other methods are necessary, but new methods also need to be thoroughly checked.

Ammonia-induced Na,K-ATPase/ouabain-mediated EGF receptor transactivation, MAPK/ERK and PI3K/AKT signaling and ROS formation cause astrocyte swelling

Ammonia toxicity is clinically important and biologically poorly understood. We reported previously that 3mM ammonia chloride (ammonia), a relevant concentration for hepatic encephalopathy studies, increases production of endogenous ouabain and activity of Na,K-ATPase in astrocytes. In addition, ammonia-induced upregulation of gene expression of α2 isoform of Na,K-ATPase in astrocytes could be inhibited by AG1478, an inhibitor of the EGF receptor (EGFR), and by PP1, an inhibitor of Src, but not by GM6001, an inhibitor of metalloproteinase and shedding of growth factor, suggesting the involvement of endogenous ouabain-induced EGF receptor transactivation. In the present cell culture study, we investigated ammonia effects on phosphorylation of EGF receptor and its intracellular signal pathway towards MAPK/ERK1/2 and PI3K/AKT; interaction between EGF receptor, α1, and α2 isoforms of Na,K-ATPase, Src, ERK1/2, AKT and caveolin-1; and relevance of these signal pathways for ammonia-induced cell swelling, leading to brain edema, an often fatal complication of ammonia toxicity. We found that (i) ammonia increases EGF receptor phosphorylation at EGFR(845) and EGFR(1068); (ii) ammonia-induced ERK1/2 and AKT phosphorylation depends on the activity of EGF receptor and Src, but not on metalloproteinase; (iii) AKT phosphorylation occurs upstream of ERK1/2 phosphorylation; (iv) ammonia stimulates association between the α1 Na,K-ATPase isoform, Src, EGF receptor, ERK1/2, AKT and caveolin-1; (v) ammonia-induced ROS production might occur later than EGFR transactivation; (vi) both ammonia induced ERK phosphorylation and ROS production can be abolished by canrenone, an inhibitor of ouabain, and (vii) ammonia-induced cell swelling depends on signaling via the Na,K-ATPase/ouabain/Src/EGF receptor/PI3K-AKT/ERK1/2, but in response to 3mM ammonia it does not appear until after 12h. Based on literature data it is suggested that the delayed appearance of the ammonia-induced swelling at this concentration reflects required ouabain-induced oxidative damage of the ion and water cotransporter NKCC1. This information may provide new therapeutic targets for treatment of hyperammonic brain disorders.

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.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.