Tonic inhibition in dentate gyrus impairs long-term potentiation and memory in an Alzheimer's [corrected] disease model

Astrocyte Reactivity in Alzheimer's Disease: Therapeutic Opportunities to Promote Repair

Astrocytes are fast climbing the ladder of importance in neurodegenerative disorders, particularly in Alzheimer's disease (AD), with the prominent presence of reactive astrocytes sur- rounding amyloid β- plaques, together with activated microglia. Reactive astrogliosis, implying morphological and molecular transformations in astrocytes, seems to precede neurodegeneration, suggesting a role in the development of the disease. Single-cell transcriptomics has recently demon- strated that astrocytes from AD brains are different from "normal" healthy astrocytes, showing dys- regulations in areas such as neurotransmitter recycling, including glutamate and GABA, and im- paired homeostatic functions. However, recent data suggest that the ablation of astrocytes in mouse models of amyloidosis results in an increase in amyloid pathology as well as in the inflammatory profile and reduced synaptic density, indicating that astrocytes mediate neuroprotective effects. The idea that interventions targeting astrocytes may have great potential for AD has therefore emerged, supported by a range of drugs and stem cell transplantation studies that have successfully shown a therapeutic effect in mouse models of AD. In this article, we review the latest reports on the role and profile of astrocytes in AD brains and how manipulation of astrocytes in animal mod- els has paved the way for the use of treatments enhancing astrocytic function as future therapeutic avenues for AD.

Positive and Negative Selective Allosteric Modulators of α5 GABAA Receptors: Effects on Emotionality, Motivation, and Motor Function in the 5xFAD Model of Alzheimer's Disease

BACKGROUND

Positive and negative allosteric modulators of α5 GABAA receptors (PAM and NAM, respectively) are worthy of investigation as putative treatments of Alzheimer's disease (AD). However, their potential to modify a dynamic range of behaviors in AD models needs to be systematically examined.

OBJECTIVE

The study aimed to assess effects of MP-III-022 as PAM and PWZ-029 as NAM on emotional reactivity, motivation, and motor function, as well as on gene expression of GABRA2, GABRA3 and GABRA5 subunit of GABAA receptors in prefrontal cortex (PFC) and hippocampus (HC) in 5xFAD mice, as an early-onset transgenic AD model.

METHODS

The 6-month-old 5xFAD transgenic and non-transgenic mice of both genders underwent a battery of reflexes and behavioral tests (sensorimotor tests, elevated plus maze, and open field) after 10-day intraperitoneal treatment with MP-III-022, PWZ-029, or solvent. The behavioral battery was followed by qPCR analysis of gene expression.

RESULTS

MP-III-022 induced a decline in motor function, while PWZ-029 further decreased emotionality of transgenic males, as compared to the transgenic control. No interfering effects on non-cognitive behavior were observed in female mice. In HC, both treatments reversed reciprocal GABRA2 and GABRA3 changes in transgenic females. In PFC, MP-III-022 decreased GABRA5 in both genders, while PWZ-029 increased GABRA2 in male transgenic animals.

CONCLUSION

Gender-dependent protracted effects of PAMs and NAMs in AD model, with detrimental impact on motor capabilities of PAM, and attenuation of emotionality elicited by NAM in transgenic males, were revealed. This favors future research of α5 GABAA receptor modulation in females as more promising.

Molecular Determinants and Pharmacological Analysis for a Class of Competitive Non-transported Bicyclic Inhibitors of the Betaine/GABA Transporter BGT1

The betaine/GABA transporter 1 (BGT1) is a member of the GABA transporter (GAT) family with still elusive function, largely due to a lack of potent and selective tool compounds. Based on modeling, we here present the design, synthesis and pharmacological evaluation of five novel conformationally restricted cyclic GABA analogs related to the previously reported highly potent and selective BGT1 inhibitor (1S,2S,5R)-5-aminobicyclo[3.1.0]hexane-2-carboxylic acid (bicyclo-GABA). Using [³H]GABA radioligand uptake assays at the four human GATs recombinantly expressed in mammalian cell lines, we identified bicyclo-GABA and its N-methylated analog (2) as the most potent and selective BGT1 inhibitors. Additional pharmacological characterization in a fluorescence-based membrane potential assay showed that bicyclo-GABA and 2 are competitive inhibitors, not substrates, at BGT1, which was validated by a Schild analysis for bicyclo-GABA (pK_B value of 6.4). To further elaborate on the selectivity profile both compounds were tested at recombinant α₁β₂γ₂ GABA_A receptors. Whereas bicyclo-GABA showed low micromolar agonistic activity, the N-methylated 2 was completely devoid of activity at GABA_A receptors. To further reveal the binding mode of bicyclo-GABA and 2 binding hypotheses of the compounds were obtained from in silico-guided mutagenesis studies followed by pharmacological evaluation at selected BGT1 mutants. This identified the non-conserved BGT1 residues Q299 and E52 as the molecular determinants driving BGT1 activity and selectivity. The binding mode of bicyclo-GABA was further validated by the introduction of activity into the corresponding GAT3 mutant L314Q (38 times potency increase cf. wildtype). Altogether, our data reveal the molecular determinants for the activity of bicyclic GABA analogs, that despite their small size act as competitive inhibitors of BGT1. These compounds may serve as valuable tools to selectively and potently target BGT1 in order to decipher its elusive pharmacological role in the brain and periphery such as the liver and kidneys.

Cognitive function improvement after fecal microbiota transplantation in Alzheimer's dementia patient: a case report

After fecal microbiota transplantation (FMT) to treat Clostridioides difficile infection (CDI), cognitive improvement is noticeable, suggesting an essential association between the gut microbiome and neural function. Although it is known that the gut microbiome is linked with cognitive function, whether FMT may lead to cognitive improvement in patients with neurodegenerative disorders remains to be elucidated. We present the case of a 90-year-old woman with Alzheimer's dementia and severe CDI who underwent FMT. Cognitive function testing (Mini-Mental State Examination, Montreal Cognitive Assessment, and Clinical Dementia Rating assessment) was performed one month before FMT and one week and one month after FMT. We collected the patients' fecal samples before FMT and 3 weeks after FMT to compare the microbiota composition. The 16S rRNA gene amplicons were analyzed using the QIIME2 platform (version 2020.2) and the Phyloseq R package. The linear discriminant analysis effect size was performed to determine the taxonomic difference between pre- and post-FMT. Functional biomarker analysis using the Kruskal-Wallis H test was performed between the pre- and post-FMT. The cognitive function tests after FMT showed an improvement compared to the tests before the procedure. FMT changed the microbiota composition in recipient feces. We found that the genera were reported to be associated with cognitive function. In addition, short-chain fatty acids were found to be significantly different between before and after FMT. This finding suggests the presence of an association between the gut microbiome and cognitive function. Further, it emphasizes the need for clinical awareness regarding the effect of FMT on the brain-gut-microbiome axis and its potential as a therapy for patients with dementia.

The Influence of Nutrition in Alzheimer's Disease: Neuroinflammation and the Microbiome vs. Transmissible Prion

Alzheimer's disease (AD) is a primary, progressive, neurodegenerative disorder. Many risk factors for the development of AD have been investigated, including nutrition. Although it has been proven that nutrition plays a role in AD, the precise mechanisms through which nutrition exerts its influence remain undefined. The object of this study is to address this issue by elucidating some of the mechanisms through which nutrition interacts with AD. This work is a qualitative systematic bibliographic review of the current literature searchable on various available databases, including PubMed, Web of Science, and Google Scholar. Our evidence comprises 31 articles selected after a systematic search process. Patients suffering with AD present a characteristic microbiome that promotes changes in microglia generating a proinflammatory state. Many similarities exist between AD and prion diseases, both in terms of symptoms and in the molecular mechanisms of pathogenesis. Changes in the composition of the gut microbiome due to dietary habits could be one of the environmental factors affecting the development of AD; however, this is probably not the only factor. Similarly, the mechanism for self-propagation of beta-amyloid seen in AD is similar to that seen in prions.

Anesthesiology: Resetting Our Sights on Long-term Outcomes: The 2020 John W. Severinghaus Lecture on Translational Science

Anesthesiologists have worked relentlessly to improve intraoperative anesthesia care. They are now well positioned to expand their horizons and address many of the longer-term adverse consequences of anesthesia and surgery. Perioperative neurocognitive disorders, chronic postoperative pain, and opioid misuse are not inevitable adverse outcomes; rather, they are preventable and treatable conditions that deserve attention. The author's research team has investigated why patients experience new cognitive deficits after anesthesia and surgery. Their animal studies have shown that anesthetic drugs trigger overactivity of "memory-blocking receptors" that persists after the drugs are eliminated, and they have discovered new strategies to preserve brain function by repurposing available drugs and developing novel therapeutics that inhibit these receptors. Clinical trials are in progress to examine the cognitive outcomes of such strategies. This work is just one example of how anesthesiologists are advancing science with the goal of improving the lives of patients.

Systematic review of human post-mortem immunohistochemical studies and bioinformatics analyses unveil the complexity of astrocyte reaction in Alzheimer's disease

AIMS

Reactive astrocytes in Alzheimer's disease (AD) have traditionally been demonstrated by increased glial fibrillary acidic protein (GFAP) immunoreactivity; however, astrocyte reaction is a complex and heterogeneous phenomenon involving multiple astrocyte functions beyond cytoskeletal remodelling. To better understand astrocyte reaction in AD, we conducted a systematic review of astrocyte immunohistochemical studies in post-mortem AD brains followed by bioinformatics analyses on the extracted reactive astrocyte markers.

METHODS

NCBI PubMed, APA PsycInfo and WoS-SCIE databases were interrogated for original English research articles with the search terms 'Alzheimer's disease' AND 'astrocytes.' Bioinformatics analyses included protein-protein interaction network analysis, pathway enrichment, and transcription factor enrichment, as well as comparison with public human -omics datasets.

RESULTS

A total of 306 articles meeting eligibility criteria rendered 196 proteins, most of which were reported to be upregulated in AD vs control brains. Besides cytoskeletal remodelling (e.g., GFAP), bioinformatics analyses revealed a wide range of functional alterations including neuroinflammation (e.g., IL6, MAPK1/3/8 and TNF), oxidative stress and antioxidant defence (e.g., MT1A/2A, NFE2L2, NOS1/2/3, PRDX6 and SOD1/2), lipid metabolism (e.g., APOE, CLU and LRP1), proteostasis (e.g., cathepsins, CRYAB and HSPB1/2/6/8), extracellular matrix organisation (e.g., CD44, MMP1/3 and SERPINA3), and neurotransmission (e.g., CHRNA7, GABA, GLUL, GRM5, MAOB and SLC1A2), among others. CTCF and ESR1 emerged as potential transcription factors driving these changes. Comparison with published -omics datasets validated our results, demonstrating a significant overlap with reported transcriptomic and proteomic changes in AD brains and/or CSF.

CONCLUSIONS

Our systematic review of the neuropathological literature reveals the complexity of AD reactive astrogliosis. We have shared these findings as an online resource available at www.astrocyteatlas.org.

Epileptic Mechanisms Shared by Alzheimer's Disease: Viewed via the Unique Lens of Genetic Epilepsy

Our recent work on genetic epilepsy (GE) has identified common mechanisms between GE and neurodegenerative diseases including Alzheimer's disease (AD). Although both disorders are seemingly unrelated and occur at opposite ends of the age spectrum, it is likely there are shared mechanisms and studies on GE could provide unique insights into AD pathogenesis. Neurodegenerative diseases are typically late-onset disorders, but the underlying pathology may have already occurred long before the clinical symptoms emerge. Pathophysiology in the early phase of these diseases is understudied but critical for developing mechanism-based treatment. In AD, increased seizure susceptibility and silent epileptiform activity due to disrupted excitatory/inhibitory (E/I) balance has been identified much earlier than cognition deficit. Increased epileptiform activity is likely a main pathology in the early phase that directly contributes to impaired cognition. It is an enormous challenge to model the early phase of pathology with conventional AD mouse models due to the chronic disease course, let alone the complex interplay between subclinical nonconvulsive epileptiform activity, AD pathology, and cognition deficit. We have extensively studied GE, especially with gene mutations that affect the GABA pathway such as mutations in GABA_A receptors and GABA transporter 1. We believe that some mouse models developed for studying GE and insights gained from GE could provide unique opportunity to understand AD. These include the pathology in early phase of AD, endoplasmic reticulum (ER) stress, and E/I imbalance as well as the contribution to cognitive deficit. In this review, we will focus on the overlapping mechanisms between GE and AD, the insights from mutations affecting GABA_A receptors, and GABA transporter 1. We will detail mechanisms of E/I imbalance and the toxic epileptiform generation in AD, and the complex interplay between ER stress, impaired membrane protein trafficking, and synaptic physiology in both GE and AD.

Reversible GABAergic dysfunction involved in hippocampal hyperactivity predicts early-stage Alzheimer disease in a mouse model

BACKGROUND

Neuronal hyperactivity related to β-amyloid (Aβ) is considered an early warning sign of Alzheimer disease (AD). Although increasing evidence supports this opinion, the underlying mechanisms are still unknown.

METHODS

Here, we recorded whole-cell synaptic currents and membrane potentials using patch clamping of acute hippocampal slices from human amyloid precursor protein (APP)/presenilin-1 transgenic (5XFAD) mice and their wild-type littermates. Biochemical methods, electron microscopic imaging, behavioral tests, and intraventricular drug delivery applied with osmotic pumps were used in this study.

RESULTS

We confirmed hyperactivity of hippocampal CA1 pyramidal neurons in 5XFAD mice using whole-cell electrophysiological recording at 2.5 months old, when local Aβ-positive plaques had not developed and only mild cognitive dysfunction occurred. We further discovered attenuated inhibitory postsynaptic currents and unchanged excitatory postsynaptic currents in CA1 pyramidal neurons, in which the intrinsic excitability was unchanged. Moreover, the density of both γ-aminobutyric acid A (GABAA) receptor subunits, α1 and γ2, was reduced in synapses of the hippocampus in transgenic mice. Intriguingly, early intervention with the GABAA receptor agonist gaboxadol reversed the hippocampal hyperactivity and modestly ameliorated cognitive performance in 5XFAD mice under our experimental conditions.

CONCLUSIONS

Inhibitory postsynaptic disruption critically contributes to abnormalities in the hippocampal network and cognition in 5XFAD mice and possibly in AD. Therefore, strengthening the GABAergic system could be a promising therapy for AD in the early stages.

Insulin differentially modulates GABA signalling in hippocampal neurons and, in an age-dependent manner, normalizes GABA-activated currents in the tg-APPSwe mouse model of Alzheimer's disease

AIM

We examined if tonic γ-aminobutyric acid (GABA)-activated currents in primary hippocampal neurons were modulated by insulin in wild-type and tg-APPSwe mice, an Alzheimer's disease (AD) model.

METHODS

GABA-activated currents were recorded in dentate gyrus (DG) granule cells and CA3 pyramidal neurons in hippocampal brain slices, from 8 to 10 weeks old (young) wild-type mice and in dorsal DG granule cells in adult, 5-6 and 10-12 (aged) months old wild-type and tg-APPSwe mice, in the absence or presence of insulin, by whole-cell patch-clamp electrophysiology.

RESULTS

In young mice, insulin (1 nmol/L) enhanced the total spontaneous inhibitory postsynaptic current (sIPSC_T ) density in both dorsal and ventral DG granule cells. The extrasynaptic current density was only increased by insulin in dorsal CA3 pyramidal neurons. In absence of action potentials, insulin enhanced DG granule cells and dorsal CA3 pyramidal neurons miniature IPSC (mIPSC) frequency, consistent with insulin regulation of presynaptic GABA release. sIPSC_T densities in DG granule cells were similar in wild-type and tg-APPSwe mice at 5-6 months but significantly decreased in aged tg-APPSwe mice where insulin normalized currents to wild-type levels. The extrasynaptic current density was increased in tg-APPSwe mice relative to wild-type littermates but, only in aged tg-APPSwe mice did insulin decrease and normalize the current.

CONCLUSION

Insulin effects on GABA signalling in hippocampal neurons are selective while multifaceted and context-based. Not only is the response to insulin related to cell-type, hippocampal axis-location, age of animals and disease but also to the subtype of neuronal inhibition involved, synaptic or extrasynaptic GABA_A receptors-activated currents.

Rapid improvement in Alzheimer's disease symptoms following fecal microbiota transplantation: a case report

Alzheimer’s disease (AD), the most common form of dementia, is a leading cause of death and a major cause of morbidity in older people. The disease is characterized by progressive memory loss, cognitive impairment, and the cerebral accumulation of amyloid-β peptide. Given the health and economic impacts of AD, treatments that target the underlying etiology of AD or modify the course of the disease are of significant interest. The gut microbiome has been increasingly implicated in the pathogenesis of several neurological diseases, including multiple sclerosis and Parkinson’s disease. Furthermore, emerging evidence has demonstrated that there are alterations in gut microbiome composition in patients with AD, suggesting involvement of the microbiome–gut–brain axis. We present symptom improvement in a patient with AD following fecal microbiota transplantation for a Clostridioides difficile infection.

Astrocytes and Adenosine A2A Receptors: Active Players in Alzheimer's Disease

Astrocytes, through their numerous processes, establish a bidirectional communication with neurons that is crucial to regulate synaptic plasticity, the purported neurophysiological basis of memory. This evidence contributed to change the classic “neurocentric” view of Alzheimer’s disease (AD), being astrocytes increasingly considered a key player in this neurodegenerative disease. AD, the most common form of dementia in the elderly, is characterized by a deterioration of memory and of other cognitive functions. Although, early cognitive deficits have been associated with synaptic loss and dysfunction caused by amyloid-β peptides (Aβ), accumulating evidences support a role of astrocytes in AD. Astrocyte atrophy and reactivity occurring at early and later stages of AD, respectively, involve morphological alterations that translate into functional changes. However, the main signals responsible for astrocytic alterations in AD and their impact on synaptic function remain to be defined. One possible candidate is adenosine, which can be formed upon extracellular catabolism of ATP released by astrocytes. Adenosine can act as a homeostatic modulator and also as a neuromodulator at the synaptic level, through the activation of adenosine receptors, mainly of A₁R and A_2AR subtypes. These receptors are also present in astrocytes, being particularly relevant in pathological conditions, to control the morphofunctional responses of astrocytes. Here, we will focus on the role of A_2AR, since they are particularly associated with neurodegeneration and also with memory processes. Furthermore, A_2AR levels are increased in the AD brain, namely in astrocytes where they can control key astrocytic functions. Thus, unveiling the role of A_2AR in astrocytes function might shed light on novel therapeutic strategies for AD.

Traumatic Brain Injury Broadly Affects GABAergic Signaling in Dentate Gyrus Granule Cells

Traumatic brain injury (TBI) causes cellular and molecular alterations that contribute to neuropsychiatric disease and epilepsy. GABAergic dysfunction figures prominently in the pathophysiology of TBI, yet the effects of TBI on tonic inhibition in hippocampus remain uncertain. We used a mouse model of severe TBI [controlled cortical impact (CCI)] to investigate GABAergic signaling in dentate gyrus granule cells (DGGCs). Basal tonic GABA currents were not affected by CCI. However, tonic currents induced by the δ subunit-selective GABA_A receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; 10 μm) were reduced by 44% in DGGCs ipsilateral to CCI (CCI-ipsi), but not in contralateral DGGCs. Reduced THIP currents were apparent one week after injury and persisted up to 15 weeks. The frequency of spontaneous IPSCs (sIPSCs) was reduced in CCI-ipsi cells, but the amplitude and kinetics of sIPSCs were unaffected. Immunohistochemical analysis showed reduced expression of GABA_A receptor δ subunits and GABA_B receptor B2 subunits after CCI, by 43% and 40%, respectively. Activation of postsynaptic GABA_B receptors caused a twofold increase in tonic currents, and this effect was markedly attenuated in CCI-ipsi cells (92% reduction). GABA_B receptor-activated K⁺ currents in DGGCs were also significantly reduced in CCI-ipsi cells, confirming a functional deficit of GABA_B receptors after CCI. Results indicate broad disruption of GABAergic signaling in DGGCs after CCI, with deficits in both phasic and tonic inhibition and GABA_B receptor function. These changes are predicted to disrupt operation of hippocampal networks and contribute to sequelae of severe TBI, including epilepsy.

Excessive Astrocytic GABA Causes Cortical Hypometabolism and Impedes Functional Recovery after Subcortical Stroke

Glucose hypometabolism in cortical structures after functional disconnection is frequently reported in patients with white matter diseases such as subcortical stroke. However, the molecular and cellular mechanisms have been poorly elucidated. Here we show, in an animal model of internal capsular infarct, that GABA-synthesizing reactive astrocytes in distant cortical areas cause glucose hypometabolism via tonic inhibition of neighboring neurons. We find that reversal of aberrant astrocytic GABA synthesis, by pharmacological inhibition and astrocyte-specific gene silencing of MAO-B, reverses the reduction in cortical glucose metabolism. Moreover, induction of aberrant astrocytic GABA synthesis by cortical injection of putrescine or adenovirus recapitulates cortical hypometabolism. Furthermore, MAO-B inhibition causes a remarkable recovery from post-stroke motor deficits when combined with a rehabilitation regimen. Collectively, our data indicate that cortical glucose hypometabolism in subcortical stroke is caused by aberrant astrocytic GABA and MAO-B inhibition and that attenuating cortical hypometabolism can be a therapeutic approach in subcortical stroke.

Alternative Targets to Fight Alzheimer's Disease: Focus on Astrocytes

The available treatments for patients affected by Alzheimer’s disease (AD) are not curative. Numerous clinical trials have failed during the past decades. Therefore, scientists need to explore new avenues to tackle this disease. In the present review, we briefly summarize the pathological mechanisms of AD known so far, based on which different therapeutic tools have been designed. Then, we focus on a specific approach that is targeting astrocytes. Indeed, these non-neuronal brain cells respond to any insult, injury, or disease of the brain, including AD. The study of astrocytes is complicated by the fact that they exert a plethora of homeostatic functions, and their disease-induced changes could be context-, time-, and disease specific. However, this complex but fervent area of research has produced a large amount of data targeting different astrocytic functions using pharmacological approaches. Here, we review the most recent literature findings that have been published in the last five years to stimulate new hypotheses and ideas to work on, highlighting the peculiar ability of palmitoylethanolamide to modulate astrocytes according to their morpho-functional state, which ultimately suggests a possible potential disease-modifying therapeutic approach for AD.

Activity- and sleep-dependent regulation of tonic inhibition by Shisa7

Tonic inhibition mediated by extrasynaptic γ-aminobutyric acid type A receptors (GABAARs) critically regulates neuronal excitability and brain function. However, the mechanisms regulating tonic inhibition remain poorly understood. Here, we report that Shisa7 is critical for tonic inhibition regulation in hippocampal neurons. In juvenile Shisa7 knockout (KO) mice, α5-GABAAR-mediated tonic currents are significantly reduced. Mechanistically, Shisa7 is crucial for α5-GABAAR exocytosis. Additionally, Shisa7 regulation of tonic inhibition requires protein kinase A (PKA) that phosphorylates Shisa7 serine 405 (S405). Importantly, tonic inhibition undergoes activity-dependent regulation, and Shisa7 is required for homeostatic potentiation of tonic inhibition. Interestingly, in young adult Shisa7 KOs, basal tonic inhibition in hippocampal neurons is unaltered, largely due to the diminished α5-GABAAR component of tonic inhibition. However, at this stage, tonic inhibition oscillates during the daily sleep/wake cycle, a process requiring Shisa7. Together, these data demonstrate that intricate signaling mechanisms regulate tonic inhibition at different developmental stages and reveal a molecular link between sleep and tonic inhibition.

Downregulation of GABA Transporter 3 (GAT3) is Associated with Deficient Oxidative GABA Metabolism in Human Induced Pluripotent Stem Cell-Derived Astrocytes in Alzheimer's Disease

Alterations in neurotransmitter homeostasis, primarily of glutamate and GABA, is strongly implicated in the pathophysiology of Alzheimer's disease (AD). Homeostasis at the synapse is maintained by neurotransmitter recycling between neurons and astrocytes. Astrocytes support neuronal transmission through glutamine synthesis, which can be derived from oxidative metabolism of GABA. However, the precise implications of astrocytic GABA metabolism in AD remains elusive. The aim of this study was to investigate astrocytic GABA metabolism in AD pathology implementing human induced pluripotent stem cells derived astrocytes. Metabolic mapping of GABA was performed with [U-¹³C]GABA stable isotopic labeling using gas chromatography coupled to mass spectrometry (GC-MS). Neurotransmitter and amino acid content was quantified via high performance liquid chromatography (HPLC) and protein expression was investigated by Western blot assay. Cell lines carrying mutations in either amyloid precursor protein (APP) or presenilin1 (PSEN-1) were used as AD models and were compared to a control cell line of the same genetic background. AD astrocytes displayed a reduced oxidative GABA metabolism mediated by a decreased uptake capacity of GABA, as GABA transporter 3 (GAT3) was downregulated in AD astrocytes compared to the controls. Interestingly, the carbon backbone of GABA in AD astrocytes was utilized to a larger extent to support glutamine synthesis compared to control astrocytes. The results strongly indicate alterations in GABA uptake and metabolism in AD astrocytes linked to reduced GABA transporter expression, hereby contributing further to neurotransmitter disturbances.

Enhancing GABAergic signaling ameliorates aberrant gamma oscillations of olfactory bulb in AD mouse models

Background

Before the deposition of amyloid-beta plaques and the onset of learning memory deficits, patients with Alzheimer’s disease (AD) experience olfactory dysfunction, typified by a reduced ability to detect, discriminate, and identify odors. Rodent models of AD, such as the Tg2576 and APP/PS1 mice, also display impaired olfaction, accompanied by aberrant in vivo or in vitro gamma rhythms in the olfactory pathway. However, the mechanistic relationships between the electrophysiological, biochemical and behavioral phenomena remain unclear.

Methods

To address the above issues in AD models, we conducted in vivo measurement of local field potential (LFP) with a combination of in vitro electro-olfactogram (EOG), whole-cell patch and field recordings to evaluate oscillatory and synaptic function and pharmacological regulation in the olfactory pathway, particularly in the olfactory bulb (OB). Levels of protein involved in excitation and inhibition of the OB were investigated by western blotting and fluorescence staining, while behavioral studies assessed olfaction and memory function.

Results

LFP measurements demonstrated an increase in gamma oscillations in the OB accompanied by altered olfactory behavior in both APP/PS1 and 3xTg mice at 3–5 months old, i.e. an age before the onset of plaque formation. Fewer olfactory sensory neurons (OSNs) and a reduced EOG contributed to a decrease in the excitatory responses of M/T cells, suggesting a decreased ability of M/T cells to trigger interneuron GABA release indicated by altered paired-pulse ratio (PPR), a presynaptic parameter. Postsynaptically, there was a compensatory increase in levels of GABA_AR α1 and β3 subunits and subsequent higher amplitude of inhibitory responses. Strikingly, the GABA uptake inhibitor tiagabine (TGB) ameliorated abnormal gamma oscillations and levels of GABA_AR subunits, suggesting a potential therapeutic strategy for early AD symptoms. These findings reveal increased gamma oscillations in the OB as a core indicator prior to onset of AD and uncover mechanisms underlying aberrant gamma activity in the OB.

Conclusions

This study suggests that the concomitant dysfunction of both olfactory behavior and gamma oscillations have important implications for early AD diagnosis: in particular, awareness of aberrant GABAergic signaling mechanisms might both aid diagnosis and suggest therapeutic strategies for olfactory damage in AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00434-7.

Brain ethanol metabolism by astrocytic ALDH2 drives the behavioural effects of ethanol intoxication

Alcohol is among the most widely used psychoactive substances worldwide. Ethanol metabolites such as acetate, thought to be primarily the result of ethanol breakdown by hepatic aldehyde dehydrogenase 2 (ALDH2), contribute to alcohol's behavioural effects and alcoholism. Here, we show that ALDH2 is expressed in astrocytes in the mouse cerebellum and that ethanol metabolism by astrocytic ALDH2 mediates behavioural effects associated with ethanol intoxication. We show that ALDH2 is expressed in astrocytes in specific brain regions and that astrocytic, but not hepatocytic, ALDH2 is required to produce ethanol-derived acetate in the mouse cerebellum. Cerebellar astrocytic ALDH2 mediates low-dose ethanol-induced elevation of GABA levels, enhancement of tonic inhibition and impairment of balance and coordination skills. Thus, astrocytic ALDH2 controls the production, cellular and behavioural effects of alcohol metabolites in a brain-region-specific manner. Our data indicate that astrocytic ALDH2 is an important, but previously under-recognized, target in the brain to alter alcohol pharmacokinetics and potentially treat alcohol use disorder.

Contribution of astrocytes to neuropathology of neurodegenerative diseases

Classically, the loss of vulnerable neuronal populations in neurodegenerative diseases was considered to be the consequence of cell autonomous degeneration of neurons. However, progress in the understanding of glial function, the availability of improved animal models recapitulating the features of the human diseases, and the development of new approaches to derive glia and neurons from induced pluripotent stem cells obtained from patients, provided novel information that altered this view. Current evidence strongly supports the notion that non-cell autonomous mechanisms contribute to the demise of neurons in neurodegenerative disorders, and glia causally participate in the pathogenesis and progression of these diseases. In addition to microglia, astrocytes have emerged as key players in neurodegenerative diseases and will be the focus of the present review. Under the influence of pathological stimuli present in the microenvironment of the diseased CNS, astrocytes undergo morphological, transcriptional, and functional changes and become reactive. Reactive astrocytes are heterogeneous and exhibit neurotoxic (A1) or neuroprotective (A2) phenotypes. In recent years, single-cell or single-nucleus transcriptome analyses unraveled new, disease-specific phenotypes beyond A1/A2. These investigations highlighted the complexity of the astrocytic responses to CNS pathology. The present review will discuss the contribution of astrocytes to neurodegenerative diseases with particular emphasis on Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and frontotemporal dementia. Some of the commonalties and differences in astrocyte-mediated mechanisms that possibly drive the pathogenesis or progression of the diseases will be summarized. The emerging view is that astrocytes are potential new targets for therapeutic interventions. A comprehensive understanding of astrocyte heterogeneity and disease-specific phenotypic complexity could facilitate the design of novel strategies to treat neurodegenerative disorders.

Environmental enrichment implies GAT-1 as a potential therapeutic target for stroke recovery

Rationale: Stroke is a leading cause of adult disability worldwide, but no drug provides functional recovery during the repair phase. Accumulating evidence demonstrates that environmental enrichment (EE) promotes stroke recovery by enhancing network excitability. However, the complexities of utilizing EE in a clinical setting limit its translation.

Methods: We used multifaceted approaches combining electrophysiology, chemogenetics, optogenetics, and floxed mice in a mouse photothrombotic stroke model to reveal the key target of EE-mediated stroke recovery.

Results: EE reduced tonic gamma-aminobutyric acid (GABA) inhibition and facilitated phasic GABA inhibition in the peri-infarct cortex, thereby promoting network excitability and stroke recovery. These beneficial effects depended on GAT-1, a GABA transporter regulating both tonic and phasic GABA signaling, as EE positively regulated GAT-1 expression, trafficking, and function. Furthermore, GAT-1 was necessary for EE-induced network plasticity, including structural neuroplasticity, input synaptic strengthening in the peri-infarct cortex, output synaptic strengthening in the corticospinal tract, and sprouting of uninjured corticospinal axons across the midline into the territory of denervated spinal cord, and functional recovery from stroke. Moreover, restoration of GAT-1 function in the peri-infarct cortex by its overexpression showed similar beneficial effects on stroke recovery as EE exposure.

Conclusion: GAT-1 is a key molecular substrate of the effects of EE on network excitability and consequent stroke recovery and can serve as a novel therapeutic target for stroke treatment during the repair phase.

Alpha-asarone Improves Cognitive Function of APP/PS1 Mice and Reducing Aβ42, P-tau and Neuroinflammation, and Promoting Neuron Survival in the Hippocampus

Alzheimer's disease (AD) is a progressive neurodegenerative disease most often characterized by memory impairment and cognitive decline. Alpha-asarone has been reported to have the potential to treat AD. Our previous studies have found that alpha-asarone improves aged rats' cognitive function by alleviating neuronal excitotoxicity via type A gamma-aminobutyric acid (GABA) receptors. GABA level's change, neuroinflammation, and dysfunctional autophagy are found to be associated with AD. However, the effect of alpha-asarone on cognitive function of APP/PS1 transgenic mice and its underlying mechanism in terms of aggregation of amyloid-β42 (Aβ₄₂) and phosphorylated tau (p-tau), glutamic acid decarboxylase (GAD) level, neuroinflammation, and autophagy are unclear. Accordingly, we attempted to explore whether alpha-asarone improves AD mice's cognitive function and alleviates pathological symptoms by regulating GAD level, inhibiting neuroinflammation, or restore autophagy. We found that alpha-asarone enhanced spatial learning memory and decreased Aβ₄₂ and p-tau levels without influencing the GAD level in APP/PS1 transgenic mice. Also, it decreased the GFAP expression and reduced pro-inflammatory cytokines levels, thus alleviating neuroinflammation. Furthermore, alpha-asarone decreased the excess number of autophagosomes and promoted hippocampal neurons' survival. In conclusion, the results confirmed the therapeutic effect of alpha-asarone on AD-related astrogliosis, dysfunctional autophagy, and neuronal damage, which indicates its great potential to treat AD.

From Hybrids to New Scaffolds: The Latest Medicinal Chemistry Goals in Multi-target Directed Ligands for Alzheimer's Disease

Alzheimer's disease (AD) is a chronic, progressive, and fatal neurodegenerative disorder affecting cognition, behavior, and function, being one of the most common causes of mental deterioration in elderly people. Once thought as being just developed because of β amyloid depositions or neurofibrillary Tau tangles, during the last decades, numerous AD-related targets have been established, the multifactorial nature of AD became evident. In this context, the one drug-one target paradigm has resulted in being inefficient in facing AD and other disorders with complex etiology, opening the field for the emergence of the multitarget approach. In this review, we highlight the recent advances within this area, emphasizing in hybridization tools of well-known chemical scaffolds endowed with pharmacological properties concerning AD, such as curcumin-, resveratrol-, chromone- and indole-. We focus mainly on well established and incipient AD therapeutic targets, AChE, BuChE, MAOs, β-amyloid deposition, 5-HT4 and Serotonin transporter, with the aim to shed light about new insights in the AD multitarget therapy.

Bridging the Metabolic Parallels Between Neurological Diseases and Cancer

Despite the many recent breakthroughs in cancer research, oncology has traditionally been seen as a distinct field from other diseases. Recently, more attention has been paid to repurposing established therapeutic strategies and targets of other diseases towards cancer treatment, with some of these attempts generating promising outcomes [1, 2]. Recent studies using advanced metabolomics technologies [3] have shown evidence of close metabolic similarities between cancer and neurological diseases. These studies have unveiled several metabolic characteristics shared by these two categories of diseases, including metabolism of glutamine, gamma-aminobutyric acid (GABA), and N-acetyl-aspartyl-glutamate (NAAG) [4-6]. The striking metabolic overlap between cancer and neurological diseases sheds light on novel therapeutic strategies for cancer treatment. For example, 2-(phosphonomethyl) pentanedioic acid (2-PMPA), one of the glutamate carboxypeptidase II (GCP II) inhibitors that prevent the conversion of NAAG to glutamate, has been shown to suppress cancer growth [6, 7]. These promising results have led to an increased interest in integrating this metabolic overlap between cancer and neurological diseases into the study of cancer metabolism. The advantages of studying this metabolic overlap include not only drug repurposing but also translating existing knowledge from neurological diseases to the field of cancer research. This chapter discusses the specific overlapping metabolic features between cancer and neurological diseases, focusing on glutamine, GABA, and NAAG metabolisms. Understanding the interconnections between cancer and neurological diseases will guide researchers and clinicians to find more effective cancer treatments.

Drinking water temperature affects cognitive function and progression of Alzheimer's disease in a mouse model

Lifestyle factors may affect mental health and play a critical role in the development of neurodegenerative diseases including Alzheimer's disease (AD). However, whether the temperatures of daily beverages have any impact on cognitive function and AD development has never been studied. In this study, we investigated the effects of daily drinking water temperatures on cognitive function and AD development and progression in mice and the underlying mechanisms. Cognitive function of mice was assessed using passive avoidance test, open field test, and Morris water maze. Wild-type Kunming mice receiving intragastric water (IW, 10 mL/kg, 2 times/day) at 0 °C for consecutive 15 days displayed significant cognitive defects accompanied by significant decrease in gain of body weight, gastric emptying rate, pepsin activity, and an increase in the energy charge in the cortex when compared with mice receiving the same amount of IW at 25 °C (a temperature mimicking most common drinking habits in human), suggesting the altered neuroenergetics may cause cognitive decline. Similarly, in the transgenic APPwse/PS1De9 familial AD mice and their age- and gender-matched wild-type C57BL/6 mice, receiving IW at 0 °C, but not at 25 °C, for 35 days caused a significant time-dependent decrease in body weight and cognitive function, accompanied by a decreased expression of PI3K, Akt, the glutamate/GABA ratio, as well as neuropathy with significant amyloid lesion in the cortex and hippocampus. All of these changes were significantly aggravated in the APPwse/PS1De9 mice than in the control C57BL/6 mice. These data demonstrate that daily beverage at 0 °C may alter brain insulin-mediated neuroenergetics, glutamate/GABA ratio, cause cognitive decline and neuropathy, and promote AD progression.

Astrocytes: a double-edged sword in neurodegenerative diseases

Astrocytes play multifaceted and vital roles in maintaining neurophysiological function of the central nervous system by regulating homeostasis, increasing synaptic plasticity, and sustaining neuroprotective effects. Astrocytes become activated as a result of inflammatory responses during the progression of pathological changes associated with neurodegenerative disorders. Reactive astrocytes (neurotoxic A1 and neuroprotective A2) are triggered during disease progression and pathogenesis due to neuroinflammation and ischemia. However, only a limited body of literature describes morphological and functional changes of astrocytes during the progression of neurodegenerative diseases. The present review investigated the detrimental and beneficial roles of astrocytes in neurodegenerative diseases reported in recent studies, as these cells have promising therapeutic potential and offer new approaches for treatment of neurodegenerative diseases.

Inhibition of a tonic inhibitory conductance in mouse hippocampal neurones by negative allosteric modulators of α5 subunit-containing γ-aminobutyric acid type A receptors: implications for treating cognitive deficits

BACKGROUND

Multiple cognitive and psychiatric disorders are associated with an increased tonic inhibitory conductance that is generated by α5 subunit-containing γ-aminobutyric acid type A (α5 GABA_A) receptors. Negative allosteric modulators that inhibit α5 GABA_A receptors (α5-NAMs) are being developed as treatments for these disorders. The effects of α5-NAMs have been studied on recombinant GABA_A receptors expressed in non-neuronal cells; however, no study has compared drug effects on the tonic conductance generated by native GABA_A receptors in neurones, which was the goal of this study.

METHODS

The effects of five α5-NAMs (basmisanil, Ono-160, L-655,708, α5IA, and MRK-016) on tonic current evoked by a low concentration of GABA were studied using whole-cell recordings in cultured mouse hippocampal neurones. Drug effects on current evoked by a saturating concentration of GABA and on miniature inhibitory postsynaptic currents (mIPSCs) were also examined.

RESULTS

The α5-NAMs caused a concentration-dependent decrease in tonic current. The potencies varied as the inhibitory concentration for 50% inhibition (IC₅₀) of basmisanil (127 nM) was significantly higher than those of the other compounds (0.4-0.8 nM). In contrast, the maximal efficacies of the drugs were similar (35.5-51.3% inhibition). The α5-NAMs did not modify current evoked by a saturating GABA concentration or mIPSCs.

CONCLUSIONS

Basmisanil was markedly less potent than the other α5-NAMs, an unexpected result based on studies of recombinant α5 GABA_A receptors. Studying the effects of α5 GABA_A receptor-selective drugs on the tonic inhibitory current in neurones could inform the selection of compounds for future clinical trials.

Astrocytic GABA Accumulation in Experimental Temporal Lobe Epilepsy

An imbalance of excitation and inhibition has been associated with the pathophysiology of epilepsy. Loss of GABAergic interneurons and/or synaptic inhibition has been shown in various epilepsy models and in human epilepsy. Despite this loss, several studies reported preserved or increased tonic GABA_A receptor-mediated currents in epilepsy, raising the question of the source of the inhibitory transmitter. We used the unilateral intracortical kainate mouse model of temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS) to answer this question. In our model we observed profound loss of interneurons in the sclerotic hippocampal CA1 region and dentate gyrus already 5 days after epilepsy induction. Consistent with the literature, the absence of interneurons caused no reduction of tonic inhibition of CA1 pyramidal neurons. In dentate granule cells the inhibitory currents were even increased in epileptic tissue. Intriguingly, immunostaining of brain sections from epileptic mice with antibodies against GABA revealed strong and progressive accumulation of the neurotransmitter in reactive astrocytes. Pharmacological inhibition of the astrocytic GABA transporter GAT3 did not affect tonic inhibition in the sclerotic hippocampus, suggesting that this transporter is not responsible for astrocytic GABA accumulation or release. Immunostaining further indicated that both decarboxylation of glutamate and putrescine degradation accounted for the increased GABA levels in reactive astrocytes. Together, our data provide evidence that the preserved tonic inhibitory currents in the epileptic brain are mediated by GABA overproduction and release from astrocytes. A deeper understanding of the underlying mechanisms may lead to new strategies for antiepileptic drug therapy.

Space-Dependent Glia-Neuron Interplay in the Hippocampus of Transgenic Models of β-Amyloid Deposition

This review is focused on the description and discussion of the alterations of astrocytes and microglia interplay in models of Alzheimer's disease (AD). AD is an age-related neurodegenerative pathology with a slowly progressive and irreversible decline of cognitive functions. One of AD's histopathological hallmarks is the deposition of amyloid beta (Aβ) plaques in the brain. Long regarded as a non-specific, mere consequence of AD pathology, activation of microglia and astrocytes is now considered a key factor in both initiation and progression of the disease, and suppression of astrogliosis exacerbates neuropathology. Reactive astrocytes and microglia overexpress many cytokines, chemokines, and signaling molecules that activate or damage neighboring cells and their mutual interplay can result in virtuous/vicious cycles which differ in different brain regions. Heterogeneity of glia, either between or within a particular brain region, is likely to be relevant in healthy conditions and disease processes. Differential crosstalk between astrocytes and microglia in CA1 and CA3 areas of the hippocampus can be responsible for the differential sensitivity of the two areas to insults. Understanding the spatial differences and roles of glia will allow us to assess how these interactions can influence the state and progression of the disease, and will be critical for identifying therapeutic strategies.

Rethinking Intellectual Disability from Neuro- to Astro-Pathology

Neurodevelopmental disorders arise from genetic and/or from environmental factors and are characterized by different degrees of intellectual disability. The mechanisms that govern important processes sustaining learning and memory, which are severely affected in intellectual disability, have classically been thought to be exclusively under neuronal control. However, this vision has recently evolved into a more integrative conception in which astroglia, rather than just acting as metabolic supply and structural anchoring for neurons, interact at distinct levels modulating neuronal communication and possibly also cognitive processes. Recently, genetic tools have made it possible to specifically manipulate astrocyte activity unraveling novel functions that involve astrocytes in memory function in the healthy brain. However, astrocyte manipulation has also underscored potential mechanisms by which dysfunctional astrocytes could contribute to memory deficits in several neurodevelopmental disorders revealing new pathogenic mechanisms in intellectual disability. Here, we review the current knowledge about astrocyte dysfunction that might contribute to learning and memory impairment in neurodevelopmental disorders, with special focus on Fragile X syndrome and Down syndrome.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

Deficient astrocyte metabolism impairs glutamine synthesis and neurotransmitter homeostasis in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) leads to cerebral accumulation of insoluble amyloid-β plaques causing synaptic dysfunction and neuronal death. Neurons rely on astrocyte-derived glutamine for replenishment of the amino acid neurotransmitter pools. Perturbations of astrocyte glutamine synthesis have been described in AD, but whether this functionally affects neuronal neurotransmitter synthesis is not known. Since the synthesis and recycling of neurotransmitter glutamate and GABA are intimately coupled to cellular metabolism, the aim of this study was to provide a functional investigation of neuronal and astrocytic energy and neurotransmitter metabolism in AD. To achieve this, we incubated acutely isolated cerebral cortical and hippocampal slices from 8-month-old female 5xFAD mice, in the presence of ¹³C isotopically enriched substrates, with subsequent gas chromatography-mass spectrometry (GC-MS) analysis. A prominent neuronal hypometabolism of [U-¹³C]glucose was observed in the hippocampal slices of the 5xFAD mice. Investigating astrocyte metabolism, using [1,2-¹³C]acetate, revealed a marked reduction in glutamine synthesis, which directly hampered neuronal synthesis of GABA. This was supported by an increased metabolism of exogenously supplied [U-¹³C]glutamine, suggesting a neuronal metabolic compensation of the reduced astrocytic glutamine supply. In contrast, astrocytic metabolism of [U-¹³C]GABA was reduced, whereas [U-¹³C]glutamate metabolism was unaffected. Finally, astrocyte de novo synthesis of glutamate and glutamine was hampered, whereas the enzymatic capacity of glutamine synthetase for ammonia fixation was maintained. Collectively, we demonstrate that deficient astrocyte metabolism leads to reduced glutamine synthesis, directly impairing neuronal GABA synthesis in the 5xFAD brain. These findings suggest that astrocyte metabolic dysfunction may be fundamental for the imbalances of synaptic excitation and inhibition in the AD brain.

Impaired Expression of GABA Signaling Components in the Alzheimer's Disease Middle Temporal Gyrus

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter, playing a central role in the regulation of cortical excitability and the maintenance of the excitatory/inhibitory (E/I) balance. Several lines of evidence point to a remodeling of the cerebral GABAergic system in Alzheimer’s disease (AD), with past studies demonstrating alterations in GABA receptor and transporter expression, GABA synthesizing enzyme activity and focal GABA concentrations in post-mortem tissue. AD is a chronic neurodegenerative disorder with a poorly understood etiology and the temporal cortex is one of the earliest regions in the brain to be affected by AD neurodegeneration. Utilizing NanoString nCounter analysis, we demonstrate here the transcriptional downregulation of several GABA signaling components in the post-mortem human middle temporal gyrus (MTG) in AD, including the GABA_A receptor α₁, α₂, α₃, α₅, β₁, β₂, β₃, δ, γ₂, γ₃, and θ subunits and the GABA_B receptor 2 (GABA_BR2) subunit. In addition to this, we note the transcriptional upregulation of the betaine-GABA transporter (BGT1) and GABA transporter 2 (GAT2), and the downregulation of the 67 kDa isoform of glutamate decarboxylase (GAD₆₇), the primary GABA synthesizing enzyme. The functional consequences of these changes require further investigation, but such alterations may underlie disruptions to the E/I balance that are believed to contribute to cognitive decline in AD.

Alterations of GABA B receptors in the APP/PS1 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the progressive decline of memory and cognitive function. The disease is characterized by the presence of amyloid plaques, tau tangles, altered inflammatory signaling, and alterations in numerous neurotransmitter signaling systems, including γ-aminobutyric acid (GABA). Given the extensive role of GABA in regulating neuronal activity, a careful investigation of GABA-related changes is needed. Further, given persistent inflammation has been demonstrated to drive AD pathology, the presence of GABA B receptor expressed on glia that serve a role regulation of the immune response adds to potential implications of altered GABA in AD. There has not previously been a systematic evaluation of GABA-related changes in an amyloid model of AD that specifically focuses on examining changes in GABA B receptors. In the present study, we examined alterations in several GABA-specific targets in the APP/PS1 mouse model at different ages. In the 4-month-old cohort, no significant deficits in spatial learning and memory or alterations in any of the GABAergic targets were observed compared with wild-type controls. However, we identified significant alterations in several GABA-related targets in the 6-month-old cohort that exhibited spatial learning deficits that include changes in glutamic acid decarboxylase 65, GABA transporter type 3, and GABA B receptors protein and mRNA levels. This was the same cohort at which learning and memory deficits and significant amyloid pathology was observed. Overall, our study provides evidence of altered GABAergic signaling in an amyloid model of AD at a time point consistent with AD-related deficits.

14,15-Epoxyeicosatrienoic Acid Alleviates Pathology in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of late-onset dementia, and there exists an unmet medical need for effective treatments for AD. The accumulation of neurotoxic amyloid-β (Aβ) plaques contributes to the pathophysiology of AD. EPHX2 encoding soluble epoxide hydrolase (sEH)-a key enzyme for epoxyeicosatrienoic acid (EET) signaling that is mainly expressed in lysosomes of astrocytes in the adult brain-is cosited at a locus associated with AD, but it is unclear whether and how it contributes to the pathophysiology of AD. In this report, we show that the pharmacologic inhibition of sEH with 1-trifluoromethoxyphenyl- 3-(1-propionylpiperidin-4-yl) urea (TPPU) or the genetic deletion of Ephx2 reduces Aβ deposition in the brains of both male and female familial Alzheimer's disease (5×FAD) model mice. The inhibition of sEH with TPPU or the genetic deletion of Ephx2 alleviated cognitive deficits and prevented astrocyte reactivation in the brains of 6-month-old male 5×FAD mice. 14,15-EET levels in the brains of these mice were also increased by sEH inhibition. In cultured adult astrocytes treated with TPPU or 14,15-EET, astrocyte Aβ clearance was increased through enhanced lysosomal biogenesis. Infusion of 14,15-EET into the hippocampus of 5×FAD mice prevented the aggregation of Aβ. Notably, a higher concentration of 14,15-EET (200 ng/ml) infusion into the hippocampus reversed Aβ deposition in the brains of 6-month-old male 5×FAD mice. These results indicate that EET signaling, especially 14,15-EET, plays a key role in the pathophysiology of AD, and that targeting this pathway is a potential therapeutic strategy for the treatment of AD.SIGNIFICANCE STATEMENT There are limited treatment options for Alzheimer's disease (AD). EPHX2 encoding soluble epoxide hydrolase (sEH) is located at a locus that is linked to late-onset AD, but its contribution to the pathophysiology of AD is unclear. Here, we demonstrate that sEH inhibition or Ephx2 deletion alleviates pathology in familial Alzheimer's disease (5×FAD) mice. Inhibiting sEH or increasing 14,15-epoxyeicosatrienoic acid (EET) enhanced lysosomal biogenesis and amyloid-β (Aβ) clearance in cultured adult astrocytes. Moreover, the infusion of 14,15-EET into the hippocampus of 5×FAD mice not only prevented the aggregation of Aβ, but also reversed the deposition of Aβ. Thus, 14,15-EET plays a key role in the pathophysiology of AD and therapeutic strategies that target this pathway may be an effective treatment.

Astroglial asthenia and loss of function, rather than reactivity, contribute to the ageing of the brain

Astroglia represent a class of heterogeneous, in form and function, cells known as astrocytes, which provide for homoeostasis and defence of the central nervous system (CNS). Ageing is associated with morphological and functional remodelling of astrocytes with a prevalence of morphological atrophy and loss of function. In particular, ageing is associated with (i) decrease in astroglial synaptic coverage, (ii) deficits in glutamate and potassium clearance, (iii) reduced astroglial synthesis of synaptogenic factors such as cholesterol, (iv) decrease in aquaporin 4 channels in astroglial endfeet with subsequent decline in the glymphatic clearance, (v) decrease in astroglial metabolic support through the lactate shuttle, (vi) dwindling adult neurogenesis resulting from diminished proliferative capacity of radial stem astrocytes, (vii) decline in the astroglial-vascular coupling and deficient blood-brain barrier and (viii) decrease in astroglial ability to mount reactive astrogliosis. Decrease in reactive capabilities of astroglia are associated with rise of age-dependent neurodegenerative diseases. Astroglial morphology and function can be influenced and improved by lifestyle interventions such as intellectual engagement, social interactions, physical exercise, caloric restriction and healthy diet. These modifications of lifestyle are paramount for cognitive longevity.

Blocking glycine receptors reduces neuroinflammation and restores neurotransmission in cerebellum through ADAM17-TNFR1-NF-κβ pathway

BACKGROUND

Chronic hyperammonemia induces neuroinflammation in cerebellum, with glial activation and enhanced activation of the TNFR1-NF-kB-glutaminase-glutamate-GABA pathway. Hyperammonemia also increases glycinergic neurotransmission. These alterations contribute to cognitive and motor impairment. Activation of glycine receptors is reduced by extracellular cGMP, which levels are reduced in cerebellum of hyperammonemic rats in vivo. We hypothesized that enhanced glycinergic neurotransmission in hyperammonemic rats (1) contributes to induce neuroinflammation and glutamatergic and GABAergic neurotransmission alterations; (2) is a consequence of the reduced extracellular cGMP levels. The aims were to assess, in cerebellum of hyperammonemic rats, (a) whether blocking glycine receptors with the antagonist strychnine reduces neuroinflammation; (b) the cellular localization of glycine receptor; (c) the effects of blocking glycine receptors on the TNFR1-NF-kB-glutaminase-glutamate-GABA pathway and microglia activation; (d) whether adding extracellular cGMP reproduces the effects of strychnine.

METHODS

We analyzed in freshly isolated cerebellar slices from control or hyperammonemic rats the effects of strychnine on activation of microglia and astrocytes, the content of TNFa and IL1b, the surface expression of ADAM17, TNFR1 and transporters, the phosphorylation levels of ERK, p38 and ADAM17. The cellular localization of glycine receptor was assessed by immunofluorescence. We analyzed the content of TNFa, IL1b, HMGB1, glutaminase, and the level of TNF-a mRNA and NF-κB in Purkinje neurons. Extracellular concentrations of glutamate and GABA were performed by in vivo microdialysis in cerebellum. We tested whether extracellular cGMP reproduces the effects of strychnine in ex vivo cerebellar slices.

RESULTS

Glycine receptors are expressed mainly in Purkinje cells. In hyperammonemic rats, enhanced glycinergic neurotransmission leads to reduced membrane expression of ADAM17, resulting in increased surface expression and activation of TNFR1 and of the associated NF-kB pathway. This increases the expression in Purkinje neurons of TNFa, IL-1b, HMGB1, and glutaminase. Increased glutaminase activity leads to increased extracellular glutamate, which increases extracellular GABA. Increased extracellular glutamate and HMGB1 potentiate microglial activation. Blocking glycine receptors with strychnine or extracellular cGMP completely prevents the above pathway in hyperammonemic rats.

CONCLUSIONS

Glycinergic neurotransmission modulates neuroinflammation. Enhanced glycinergic neurotransmission in hyperammonemia would be due to reduced extracellular cGMP. These results shed some light on possible new therapeutic target pathways for pathologies associated to neuroinflammation.

Astrocytes Control Sensory Acuity via Tonic Inhibition in the Thalamus

Sensory discrimination is essential for survival. However, how sensory information is finely controlled in the brain is not well defined. Here, we show that astrocytes control tactile acuity via tonic inhibition in the thalamus. Mechanistically, diamine oxidase (DAO) and the subsequent aldehyde dehydrogenase 1a1 (Aldh1a1) convert putrescine into GABA, which is released via Best1. The GABA from astrocytes inhibits synaptically evoked firing at the lemniscal synapses to fine-tune the dynamic range of the stimulation-response relationship, the precision of spike timing, and tactile discrimination. Our findings reveal a novel role of astrocytes in the control of sensory acuity through tonic GABA release.

Blocking Astrocytic GABA Restores Synaptic Plasticity in Prefrontal Cortex of Rat Model of Depression

A decrease in synaptic plasticity and/or a change in excitation/inhibition balance have been suggested as mechanisms underlying major depression disorder. However, given the crucial role of astrocytes in balancing synaptic function, particular attention should be given to the contribution of astrocytes in these mechanisms, especially since previous findings show that astrocytes are affected and exhibit reactive-like features in depression. Moreover, it has been shown that reactive astrocytes increase the synthesis and release of GABA, contributing significantly to tonic GABA inhibition. In this study we found decreased plasticity and increased tonic GABA inhibition in the prelimbic area in acute slices from the medial prefrontal cortex in the Flinders Sensitive Line (FSL) rat model of depression. The tonic inhibition can be reduced by either blocking astrocytic intracellular Ca²⁺ signaling or by reducing astrocytic GABA through inhibition of the synthesizing enzyme MAO-B with Selegiline. Blocking GABA synthesis also restores the impaired synaptic plasticity in the FSL prefrontal cortex, providing a new antidepressant mechanism of Selegiline.

An Astrocytic Influence on Impaired Tonic Inhibition in Hippocampal CA1 Pyramidal Neurons in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disease caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. Although altered interneuron development and function are clearly demonstrated in RTT mice, a particular mode of inhibition, tonic inhibition, has not been carefully examined. We report here that tonic inhibition is significantly reduced in pyramidal neurons in the CA1 region of the hippocampus in mice where Mecp2 is deleted either in all cells or specifically in astrocytes. Since no change is detected in the level of GABA receptors, such a reduction in tonic inhibition is likely a result of decreased ambient GABA level in the extracellular space. Consistent with this explanation, we observed increased expression of a GABA transporter, GABA transporter 3 (GAT3), in the hippocampus of the Mecp2 KO mice, as well as a corresponding increase of GAT3 current in hippocampal astrocytes. These phenotypes are relevant to RTT because pharmacological blockage of GAT3 can normalize tonic inhibition and intrinsic excitability in CA1 pyramidal neurons, and rescue the phenotype of increased network excitability in acute hippocampal slices from the Mecp2 KO mice. Finally, chronic administration of a GAT3 antagonist improved a composite symptom score and extended lifespan in the Mecp2 KO mice. Only male mice were used in this study. These results not only advance our understanding of RTT etiology by defining a new neuronal phenotype and revealing how it can be influenced by astrocytic alterations, but also reveal potential targets for intervention.SIGNIFICANCE STATEMENT Our study reports a novel phenotype of reduced tonic inhibition in hippocampal CA1 pyramidal neurons in the Rett syndrome mice, reveal a potential mechanism of increased GABA transporter expression/activity in the neighboring astrocytes, describe a disease-relevant consequence in hyperexcitability, and provide preliminary evidence that targeting this phenotype may slow down disease progression in Rett syndrome mice. These results help our understanding of the disease etiology and identify a new therapeutic target for treating Rett syndrome.

GABAergic Inhibitory Interneuron Deficits in Alzheimer's Disease: Implications for Treatment

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized clinically by severe cognitive deficits and pathologically by amyloid plaques, neuronal loss, and neurofibrillary tangles. Abnormal amyloid β-protein (Aβ) deposition in the brain is often thought of as a major initiating factor in AD neuropathology. However, gamma-aminobutyric acid (GABA) inhibitory interneurons are resistant to Aβ deposition, and Aβ decreases synaptic glutamatergic transmission to decrease neural network activity. Furthermore, there is now evidence suggesting that neural network activity is aberrantly increased in AD patients and animal models due to functional deficits in and decreased activity of GABA inhibitory interneurons, contributing to cognitive deficits. Here we describe the roles played by excitatory neurons and GABA inhibitory interneurons in Aβ-induced cognitive deficits and how altered GABA interneurons regulate AD neuropathology. We also comprehensively review recent studies on how GABA interneurons and GABA receptors can be exploited for therapeutic benefit. GABA interneurons are an emerging therapeutic target in AD, with further clinical trials urgently warranted.

Extensive astrocyte metabolism of γ-aminobutyric acid (GABA) sustains glutamine synthesis in the mammalian cerebral cortex

Synaptic transmission is closely linked to brain energy and neurotransmitter metabolism. However, the extent of brain metabolism of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and the relative metabolic contributions of neurons and astrocytes, are yet unknown. The present study was designed to investigate the functional significance of brain GABA metabolism using isolated mouse cerebral cortical slices and slices of neurosurgically resected neocortical human tissue of the temporal lobe. By using dynamic isotope labeling, with [¹⁵ N]GABA and [U-¹³ C]GABA as metabolic substrates, we show that both mouse and human brain slices exhibit a large capacity for GABA metabolism. Both the nitrogen and the carbon backbone of GABA strongly support glutamine synthesis, particularly in the human cerebral cortex, indicative of active astrocytic GABA metabolism. This was further substantiated by pharmacological inhibition of the primary astrocytic GABA transporter subtype 3 (GAT3), by (S)-SNAP-5114 or 1-benzyl-5-chloro-2,3-dihydro-1H-indole-2,3-dione (compound 34), leading to significant reductions in oxidative GABA carbon metabolism. Interestingly, this was not the case when tiagabine was used to specifically inhibit GAT1, which is predominantly found on neurons. Finally, we show that acute GABA exposure does not directly stimulate glycolytic activity nor oxidative metabolism in cultured astrocytes, but can be used as an additional substrate to enhance uncoupled respiration. These results clearly show that GABA is actively metabolized in astrocytes, particularly for the synthesis of glutamine, and challenge the current view that synaptic GABA homeostasis is maintained primarily by presynaptic recycling.

GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease

OBJECTIVE

To propose a new hypothesis that GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease (AD).

BACKGROUND

Synaptic dysfunction and E/I imbalance emerge decades before the appearance of cognitive decline in AD patients, which contribute to neurodegeneration. Initially, E/I imbalance was thought to occur first, due to dysfunction of the glutamatergic and cholinergic systems. However, new evidence has demonstrated that the GABAergic system, the counterpart of E/I balance and the major inhibitory neurotransmitter system in the central nervous system, is altered enormously and that this contributes to E/I imbalance and further AD pathogenesis.

NEW HYPOTHESIS

Alterations to the GABAergic system, induced by multiple AD pathogenic or risk factors, contribute to E/I imbalance and AD pathogenesis.

MAJOR CHALLENGES FOR THE HYPOTHESIS

This GABAergic hypothesis accounts for many critical questions and common challenges confronting a new hypothesis of AD pathogenesis. More specifically, it explains why amyloid beta (Aβ), β-secretase (BACE1), apolipoprotein E4 gene (APOE ε4), hyperactive glia cells, contributes to AD pathogenesis and why age and sex are the risk factors of AD. GABAergic dysfunction promotes the spread of Aβ pathology throughout the AD brain and associated cognitive impairments, and the induction of dysfunction induced by these varied risk factors shares this common neurobiology leading to E/I imbalance. In turn, some of these factors exacerbate GABAergic dysfunction and E/I imbalance. Moreover, the GABAergic system modulates various brain functions and thus, the GABAergic hypothesis accounts for nonamnestic manifestations. Furthermore, corrections of E/I balance through manipulation of GABAergic functions have shown positive outcomes in preclinical and clinical studies, suggesting the potential of the GABAergic system as a therapeutic target in AD.

LINKAGE TO OTHER MAJOR THEORIES

Dysfunction of the GABAergic system is induced by multiple critical signaling pathways, which include the existing major theories of AD pathogenesis, such as the Aβ and neuroinflammation hypotheses. In a new perspective, this GABAergic hypothesis accounts for the E/I imbalance and related excitotoxicity, which contribute to cognitive decline and AD pathogenesis. Therefore, the GABAergic system could be a key target to restore, at least partially, the E/I balance and cognitive function in AD patients.

The Accumulation of Tau-Immunoreactive Hippocampal Granules and Corpora Amylacea Implicates Reactive Glia in Tau Pathogenesis during Aging

The microtubule-associated tau protein forms pathological inclusions that accumulate in an age-dependent manner in tauopathies including Alzheimer's disease (AD). Since age is the major risk factor for AD, we examined endogenous tau species that evolve during aging in physiological and diseased conditions. In aged mouse brain, we found tau-immunoreactive clusters embedded within structures that are reminiscent of periodic acid-Schiff (PAS) granules. We showed that PAS granules harbor distinct tau species that are more prominent in 3xTg-AD mice. Epitope profiling revealed hypo-phosphorylated rather than hyper-phosphorylated tau commonly observed in tauopathies. High-resolution imaging and 3D reconstruction suggest a link between tau clusters, reactive astrocytes, and microglia, indicating that early tau accumulation may promote neuroinflammation during aging. Using postmortem human brain, we identified tau as a component of corpora amylacea (CA), age-related structures that are functionally analogous to PAS granules. Overall, our study supports neuroimmune dysfunction as a precipitating event in tau pathogenesis.

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Tau is present in mouse hippocampal granules and human corpora amylacea

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Tau accumulates with age in hippocampal granules and is accelerated in 3xTg-AD mice

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Tau immunoreactive corpora amylacea are present in Alzheimer's disease brain

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Age-related tau deposits are associated with reactive astrocytes

An 5 GABAA Receptor Inverse Agonist, 5IA, Attenuates Amyloid Beta-Induced Neuronal Death in Mouse Hippocampal Cultures

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder for which no cognition-restoring therapies exist. Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the brain. Increasing evidence suggests a remodeling of the GABAergic system in AD, which might represent an important therapeutic target. An inverse agonist of α5 subunit-containing GABAA receptors (α5GABAARs), 3-(5-Methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3–a]phthalazine (α5IA) has cognition-enhancing properties. This study aimed to characterize the effects of α5IA on amyloid beta (Aβ_1–42)-induced molecular and cellular changes. Mouse primary hippocampal cultures were exposed to either Aβ_1-42 alone, or α5IA alone, α5IA with Aβ_1–42 or vehicle alone, and changes in cell viability and mRNA expression of several GABAergic signaling components were assessed. Treatment with 100 nM of α5IA reduced Aβ_1–42-induced cell loss by 23.8% (p < 0.0001) after 6 h and by 17.3% after 5 days of treatment (p < 0.0001). Furthermore, we observed an Aβ_1-42-induced increase in ambient GABA levels, as well as upregulated mRNA expression of the GABAAR α2,α5,β2/3 subunits and the GABABR R1 and R2 subunits. Such changes in GABARs expression could potentially disrupt inhibitory neurotransmission and normal network activity. Treatment with α5IA restored Aβ_1-42-induced changes in the expression of α5GABAARs. In summary, this compound might hold neuroprotective potential and represent a new therapeutic avenue for AD.

Structure modeling of γ-aminobutyric acid transporters - Molecular basics of ligand selectivity

γ-Aminobutyric acid transporters are responsible for regulating the GABA level in the synaptic cleft. In this way, they affect GABA-ergic transmission which is important for the proper functioning of the central nervous system. The exact structure of GABA transporters is still unknown, which hinders the design of new, potent and selective inhibitors. For these reasons, we decided to create models of all types of human gamma-aminobutyric acid transporters. They were built based on crystal structures of related proteins from the SLC6 family using homology modeling methods. The reliability of the received models has been confirmed by a number of tools assessing the quality of protein models. To determine the ligand binding mode and indicate the amino acids responsible for selectivity, docking studies and molecular dynamics simulations were performed. The amino acids lining the bottom of the main binding site have a major impact on the selective ligand binding. In addition, an important element is the non-helical fragment of the transmembrane domain 10, and several amino acids within the vestibule of the transporters, which affect its volume. To check whether obtained models are suitable to distinguish active compounds from inactive ones, enrichment plots were prepared. Results suggest that our models may be useful in the search for new inhibitors of GABA transporters of the desired selectivity.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

Tonic GABAA Conductance Favors Spike-Timing-Dependent over Theta-Burst-Induced Long-Term Potentiation in the Hippocampus

Synaptic plasticity is triggered by different patterns of network activity. Here, we investigated how LTP in CA3-CA1 synapses induced by different stimulation patterns is affected by tonic GABA_A conductances in rat hippocampal slices. Spike-timing-dependent LTP was induced by pairing Schaffer collateral stimulation with antidromic stimulation of CA1 pyramidal neurons. Theta-burst-induced LTP was induced by theta-burst stimulation of Schaffer collaterals. We mimicked increased tonic GABA_A conductance by bath application of 30 μm GABA. Surprisingly, tonic GABA_A conductance selectively suppressed theta-burst-induced LTP but not spike-timing-dependent LTP. We combined whole-cell patch-clamp electrophysiology, two-photon Ca²⁺ imaging, glutamate uncaging, and mathematical modeling to dissect the mechanisms underlying these differential effects of tonic GABA_A conductance. We found that Ca²⁺ transients during pairing of an action potential with an EPSP were less sensitive to tonic GABA_A conductance-induced shunting inhibition than Ca²⁺ transients induced by EPSP burst. Our results may explain how different forms of memory are affected by increasing tonic GABA_A conductances under physiological or pathologic conditions, as well as under the influence of substances that target extrasynaptic GABA_A receptors (e.g., neurosteroids, sedatives, antiepileptic drugs, and alcohol).SIGNIFICANCE STATEMENT Brain activity is associated with neuronal firing and synaptic signaling among neurons. Synaptic plasticity represents a mechanism for learning and memory. However, some neurotransmitters that escape the synaptic cleft or are released by astrocytes can target extrasynaptic receptors. Extrasynaptic GABA_A receptors mediate tonic conductances that reduce the excitability of neurons by shunting. This results in the decreased ability for neurons to fire action potentials, but when action potentials are successfully triggered, tonic conductances are unable to reduce them significantly. As such, tonic GABA_A conductances have minimal effects on spike-timing-dependent synaptic plasticity while strongly attenuating the plasticity evoked by EPSP bursts. Our findings shed light on how changes in tonic conductances can selectively affect different forms of learning and memory.

Canine distemper virus induces downregulation of GABAA,GABAB, and GAT1 expression in brain tissue of dogs

The aim of the study was to determine the expression profiles of GABA_A, GABA_B, and GAT1 using RT-PCR and the immunoreactivity of GAT1 via immunohistochemical and immunofluorescence assays in CDV-infected brain tissue of dogs. For this purpose, dogs with CDV and dogs without CDV were selected. The mRNA transcript levels of GABA_A, GABA_B, and GAT1 were significantly downregulated in brain tissue in the CDV-infected group as compared with that in non-CDV-infected brain tissue in the control group (p < 0.01, p < 0.001). In addition, the immunoreactivity of GAT1 in CDV-infected brain tissue was significantly lower than in the uninfected group (p < 0.05). We conclude that one of the main causes of myoclonus in CDV infections may be the blockage of postsynaptic inhibition in neurons or a lack of metabolism of GABA. In addition, a GABA neurotransmission imbalance could play a role in demyelination in CDV infections.