Dynamic microglia alterations associate with hippocampal network impairments: A turning point in amyloid pathology progression.

Alzheimer's disease is a progressive neurological disorder causing memory loss and cognitive decline. The underlying causes of cognitive deterioration and neurodegeneration remain unclear, leading to a lack of effective strategies to prevent dementia. Recent evidence highlights the role of neuroinflammation, particularly involving microglia, in Alzheimer's disease onset and progression. Characterizing the initial phase of Alzheimer's disease can lead to the discovery of new biomarkers and therapeutic targets, facilitating timely interventions for effective treatments. We used the AppNL-G-F knock-in mouse model, which resembles the amyloid pathology and neuroinflammatory characteristics of Alzheimer's disease, to investigate the transition from a pre-plaque to an early plaque stage with a combined functional and molecular approach. Our experiments show a progressive decrease in the power of cognition-relevant hippocampal gamma oscillations during the early stage of amyloid pathology, together with a modification of fast-spiking interneuron intrinsic properties and postsynaptic input. Consistently, transcriptomic analyses revealed that these effects are accompanied by changes in synaptic function-associated pathways. Concurrently, homeostasis- and inflammatory-related microglia signature genes were downregulated. Moreover, we found a decrease in Iba1-positive microglia in the hippocampus that correlates with plaque aggregation and neuronal dysfunction. Collectively, these findings support the hypothesis that microglia play a protective role during the early stages of amyloid pathology by preventing plaque aggregation, supporting neuronal homeostasis, and overall preserving the oscillatory network's functionality. These results suggest that the early alteration of microglia dynamics could be a pivotal event in the progression of Alzheimer's disease, potentially triggering plaque deposition, impairment of fast-spiking interneurons, and the breakdown of the oscillatory circuitry in the hippocampus.

Timing to be precise? An overview of spike timing-dependent plasticity, brain rhythmicity, and glial cells interplay within neuronal circuits.

In the mammalian brain information processing and storage rely on the complex coding and decoding events performed by neuronal networks. These actions are based on the computational ability of neurons and their functional engagement in neuronal assemblies where precise timing of action potential firing is crucial. Neuronal circuits manage a myriad of spatially and temporally overlapping inputs to compute specific outputs that are proposed to underly memory traces formation, sensory perception, and cognitive behaviors. Spike-timing-dependent plasticity (STDP) and electrical brain rhythms are suggested to underlie such functions while the physiological evidence of assembly structures and mechanisms driving both processes continues to be scarce. Here, we review foundational and current evidence on timing precision and cooperative neuronal electrical activity driving STDP and brain rhythms, their interactions, and the emerging role of glial cells in such processes. We also provide an overview of their cognitive correlates and discuss current limitations and controversies, future perspectives on experimental approaches, and their application in humans.

Impaired spike-gamma coupling of area CA3 fast-spiking interneurons as the earliest functional impairment in the AppNL-G-F mouse model of Alzheimer's disease.

In Alzheimer's disease (AD) the accumulation of amyloid-ß (Aß) correlates with degradation of cognition-relevant gamma oscillations. The gamma rhythm relies on proper neuronal spike-gamma coupling, specifically of fast-spiking interneurons (FSN). Here we tested the hypothesis that decrease in gamma power and FSN synchrony precede amyloid plaque deposition and cognitive impairment in AppNL-G-F knock-in mice (AppNL-G-F). The aim of the study was to evaluate the amyloidogenic pathology progression in the novel AppNL-G-F mouse model using in vitro electrophysiological network analysis. Using patch clamp of FSNs and pyramidal cells (PCs) with simultaneous gamma oscillation recordings, we compared the activity of the hippocampal network of wild-type mice (WT) and the AppNL-G-F mice at four disease stages (1, 2, 4, and 6 months of age). We found a severe degradation of gamma oscillation power that is independent of, and precedes Aß plaque formation, and the cognitive impairment reported previously in this animal model. The degradation correlates with increased Aß1-42 concentration in the brain. Analysis on the cellular level showed an impaired spike-gamma coupling of FSN from 2 months of age that correlates with the degradation of gamma oscillations. From 6 months of age PC firing becomes desynchronized also, correlating with reports in the literature of robust Aß plaque pathology and cognitive impairment in the AppNL-G-F mice. This study provides evidence that impaired FSN spike-gamma coupling is one of the earliest functional impairment caused by the amyloidogenic pathology progression likely is the main cause for the degradation of gamma oscillations and consequent cognitive impairment. Our data suggests that therapeutic approaches should be aimed at restoring normal FSN spike-gamma coupling and not just removal of Aß.

Bri2 BRICHOS chaperone rescues impaired fast-spiking interneuron behavior and neuronal network dynamics in an AD mouse model in vitro.

Synchronized and properly balanced electrical activity of neurons is the basis for the brain's ability to process information, to learn, and to remember. In Alzheimer's disease (AD), which causes cognitive decline in patients, this synchronization and balance is disturbed by the accumulation of neuropathological biomarkers such as amyloid-beta peptide (Aß42). Failure of Aß42 clearance mechanisms as well as desynchronization of crucial neuronal classes such as fast-spiking interneurons (FSN) are root causes for the disruption of the cognition-relevant gamma brain rhythm (30-80 Hz) and consequent cognitive impairment observed in AD. Here we show that recombinant BRICHOS molecular chaperone domains from ProSP-C or Bri2, which interfere with Aß42 aggregation, can rescue the gamma rhythm. We demonstrate that Aß42 progressively decreases gamma oscillation power and rhythmicity, disrupts the inhibition/excitation balance in pyramidal cells, and desynchronizes FSN firing during gamma oscillations in the hippocampal CA3 network of mice. Application of the more efficacious Bri2 BRICHOS chaperone rescued the cellular and neuronal network performance from all ongoing Aß42-induced functional impairments. Collectively, our findings offer critical missing data to explain the importance of FSN for normal network function and underscore the therapeutic potential of Bri2 BRICHOS to rescue the disruption of cognition-relevant brain rhythms in AD.

Modulation of Kv3.1/Kv3.2 promotes gamma oscillations by rescuing Aß-induced desynchronization of fast-spiking interneuron firing in an AD mouse model in vitro.

KEY POINTS: Gamma oscillations (30-80 Hz) are important for cognitive functions and depend on the synchronized activity of fast-spiking interneurons (FSN), which is crucial for network stability. Gamma oscillations are degraded in Alzheimer's disease (AD) patients exhibiting cognitive impairment, with the degree of cognitive decline correlating with the severity of gamma disruption in response to neurotoxic amyloid-beta peptide (Aß). Small molecule compounds EX15 and RE01 modulate Kv3.1/Kv3.2 potassium channels on FSN, resulting in faster activation kinetics and increased firing frequency, suggesting direct consequences for cognition-relevant gamma oscillations, particularly in a situation where network activity is pathologically compromised in the presence of neurotoxic Aß. Using electrophysiological techniques in an in vitro AD model, we found a significant effect of EX15 and RE01 with respect to counteracting toxic Aß effects on neuronal dynamics, advocating for targeting FSN activity to rescue cognitive performance from impairment caused by neurodegenerative triggers. ABSTRACT: Rhythmic electrical activity in neuronal networks such as gamma oscillations (30-80 Hz) underlies cognitive functions such as sensory perception, attention and memory. Gamma oscillations are disrupted in Alzheimer's disease (AD) patients and animal AD models, with the severity of cognitive decline correlating with the degree of rhythm disruption. Misfolded amyloid-ß peptide (Aß) is assumed to be a key trigger of AD pathology and has been show to de-synchronize action potential firing in fast-spiking interneurons (FSN), which is crucial for entraining neuronal network activity into the gamma rhythm. The synchronizing activity of FSN therefore has become one of the most suitable targets to counteract disease-driven degradation of gamma oscillations and consequent cognitive decline. EX15 and RE01 are small-molecule compounds that modulate Kv3.1/Kv3.2 potassium channels, resulting in faster activation kinetics and increased FSN firing frequency. In the present study, we investigated the potential pro-cognitive effects of EX15 and RE01 by testing their ability to modulate FSN activity during ongoing gamma oscillations in normal and Aß-disrupted network states in mouse hippocampus in vitro. In the compromised, but not the uncompromised, network state with gamma oscillations partially disrupted by Aß, both compounds improve gamma oscillation regularity by promoting re-synchronization of FSN action potential firing. Our data suggest a therapeutic potential for compounds such as EX15 and RE01, which can rescue normal action potential firing parameters in FSN, in the search for disease-modifying drug candidates counteracting the progressive dysfunction of neuronal network dynamics that underlies the cognitive impairment typical of AD and other cognitive brain disorders.

Human cerebrospinal fluid promotes spontaneous gamma oscillations in the hippocampus in vitro.

Gamma oscillations (30-80 Hz) are fast network activity patterns frequently linked to cognition. They are commonly studied in hippocampal brain slices in vitro, where they can be evoked via pharmacological activation of various receptor families. One limitation of this approach is that neuronal activity is studied in a highly artificial extracellular fluid environment, as provided by artificial cerebrospinal fluid (aCSF). Here, we examine the influence of human cerebrospinal fluid (hCSF) on kainate-evoked and spontaneous gamma oscillations in mouse hippocampus. We show that hCSF, as compared to aCSF of matched electrolyte and glucose composition, increases the power of kainate-evoked gamma oscillations and induces spontaneous gamma activity in areas CA3 and CA1 that is reversed by washout. Bath application of atropine entirely abolished hCSF-induced gamma oscillations, indicating critical contribution from muscarinic acetylcholine receptor-mediated signaling. In separate whole-cell patch clamp recordings from rat hippocampus, hCSF increased theta resonance frequency and strength in pyramidal cells along with enhancement of h-current (Ih ) amplitude. We found no evidence of intrinsic gamma frequency resonance at baseline (aCSF) among fast-spiking interneurons, and this was not altered by hCSF. However, hCSF increased the excitability of fast-spiking interneurons, which likely contributed to gamma rhythmogenesis. Our findings show that hCSF promotes network gamma oscillations in the hippocampus in vitro and suggest that neuromodulators distributed in CSF could have significant influence on neuronal network activity in vivo.

Chronic inhibition of brain glycolysis initiates epileptogenesis.

Metabolic abnormalities found in epileptogenic tissue provide considerable evidence of brain hypometabolism, while major risk factors for acquired epilepsy all share brain hypometabolism as one common outcome, suggesting that a breakdown of brain energy homeostasis may actually precede epileptogenesis. However, a causal link between deficient brain energy metabolism and epilepsy initiation has not been yet established. To address this issue we developed an in vivo model of chronic energy hypometabolism by daily intracerebroventricular (i.c.v.) injection of the nonmetabolizable glucose analog 2-deoxy-D-glucose (2-DG) and also investigated acute effects of 2-DG on the cellular level. In hippocampal slices, acute glycolysis inhibition by 2-DG (by about 35%) led to contrasting effects on the network: a downregulation of excitatory synaptic transmission together with a depolarization of neuronal resting potential and a decreased drive of inhibitory transmission. Therefore, the potential acute effect of 2-DG on network excitability depends on the balance between these opposing pre- and postsynaptic changes. In vivo, we found that chronic 2-DG i.c.v. application (estimated transient inhibition of brain glycolysis under 14%) for a period of 4 weeks induced epileptiform activity in initially healthy male rats. Our results suggest that chronic inhibition of brain energy metabolism, characteristics of the well-established risk factors of acquired epilepsy, and specifically a reduction in glucose utilization (typically observed in epileptic patients) can initiate epileptogenesis. © 2017 Wiley Periodicals, Inc.

Dementia-related Bri2 BRICHOS is a versatile molecular chaperone that efficiently inhibits Aß42 toxicity in Drosophila.

Formation of fibrils of the amyloid-ß peptide (Aß) is suggested to play a central role in neurodegeneration in Alzheimer's disease (AD), for which no effective treatment exists. The BRICHOS domain is a part of several disease-related proproteins, the most studied ones being Bri2 associated with familial dementia and prosurfactant protein C (proSP-C) associated with lung amyloid. BRICHOS from proSP-C has been found to be an efficient inhibitor of Aß aggregation and toxicity, but its lung-specific expression makes it unsuited to target in AD. Bri2 is expressed in the brain, affects processing of Aß precursor protein, and increased levels of Bri2 are found in AD brain, but the specific role of its BRICHOS domain has not been studied in vivo Here, we find that transgenic expression of the Bri2 BRICHOS domain in the Drosophila central nervous system (CNS) or eyes efficiently inhibits Aß42 toxicity. In the presence of Bri2 BRICHOS, Aß42 is diffusely distributed throughout the mushroom bodies, a brain region involved in learning and memory, whereas Aß42 expressed alone or together with proSP-C BRICHOS forms punctuate deposits outside the mushroom bodies. Recombinant Bri2 BRICHOS domain efficiently prevents Aß42-induced reduction in γ-oscillations in hippocampal slices. Finally, Bri2 BRICHOS inhibits several steps in the Aß42 fibrillation pathway and prevents aggregation of heat-denatured proteins, indicating that it is a more versatile chaperone than proSP-C BRICHOS. These findings suggest that Bri2 BRICHOS can be a physiologically relevant chaperone for Aß in the CNS and needs to be further investigated for its potential in AD treatment.

Amyloid-ß-induced action potential desynchronization and degradation of hippocampal gamma oscillations is prevented by interference with peptide conformation change and aggregation.

The amyloid-ß hypothesis of Alzheimer's Disease (AD) focuses on accumulation of amyloid-ß peptide (Aß) as the main culprit for the myriad physiological changes seen during development and progression of AD including desynchronization of neuronal action potentials, consequent development of aberrant brain rhythms relevant for cognition, and final emergence of cognitive deficits. The aim of this study was to elucidate the cellular and synaptic mechanisms underlying the Aß-induced degradation of gamma oscillations in AD, to identify aggregation state(s) of Aß that mediate the peptides neurotoxicity, and to test ways to prevent the neurotoxic Aß effect. We show that Aß(1-42) in physiological concentrations acutely degrades mouse hippocampal gamma oscillations in a concentration- and time-dependent manner. The underlying cause is an Aß-induced desynchronization of action potential generation in pyramidal cells and a shift of the excitatory/inhibitory equilibrium in the hippocampal network. Using purified preparations containing different aggregation states of Aß, as well as a designed ligand and a BRICHOS chaperone domain, we provide evidence that the severity of Aß neurotoxicity increases with increasing concentration of fibrillar over monomeric Aß forms, and that Aß-induced degradation of gamma oscillations and excitatory/inhibitory equilibrium is prevented by compounds that interfere with Aß aggregation. Our study provides correlative evidence for a link between Aß-induced effects on synaptic currents and AD-relevant neuronal network oscillations, identifies the responsible aggregation state of Aß and proofs that strategies preventing peptide aggregation are able to prevent the deleterious action of Aß on the excitatory/inhibitory equilibrium and on the gamma rhythm.

Interconnection and synchronization of neuronal populations in the mouse medial septum/diagonal band of Broca.

The medial septum/diagonal band of Broca (MS/DBB) is crucial for hippocampal theta rhythm generation (4-12 Hz). However, the mechanisms behind theta rhythmogenesis are still under debate. The MS/DBB consists, in its majority, of three neuronal populations that use acetylcholine, GABA, or glutamate as neurotransmitter. While the firing patterns of septal neurons enable the MS/DBB to generate rhythmic output critical for the generation of the hippocampal theta rhythm, the ability to synchronize these action potentials is dependent on the interconnectivity between the three major MS/DBB neuronal populations, yet little is known about intraseptal connections. Here we assessed the connectivity between pairs of MS/DBB neurons with paired patch-clamp recordings. We found that glutamatergic and GABAergic neurons provide intraseptal connections and produce sizable currents in MS/DBB postsynaptic cells. We also analyzed linear and nonlinear relationships between the action potentials fired by pairs of neurons belonging to various MS/DBB neuronal populations. Our results show that while the synchrony index for action potential firing was significantly higher in pairs of GABAergic neurons, coherence of action potential firing in the theta range was similarly low in all pairs analyzed. Recurrence analysis demonstrated that individual action potentials were more recurrent in cholinergic neurons than in other cell types. Implementing sparse connectivity in a computer model of the MS/DBB network reproduced our experimental data. We conclude that the interplay between the intrinsic membrane properties of different MS/DBB neuronal populations and the connectivity among these populations underlie the ability of the MS/DBB network to critically contribute to hippocampal theta rhythmogenesis.

Neuregulin and dopamine modulation of hippocampal gamma oscillations is dependent on dopamine D4 receptors.

The neuregulin/ErbB signaling network is genetically associated with schizophrenia and modulates hippocampal γ oscillations--a type of neuronal network activity important for higher brain processes and altered in psychiatric disorders. Because neuregulin-1 (NRG-1) dramatically increases extracellular dopamine levels in the hippocampus, we investigated the relationship between NRG/ErbB and dopamine signaling in hippocampal γ oscillations. Using agonists for different D1- and D2-type dopamine receptors, we found that the D4 receptor (D4R) agonist PD168077, but not D1/D5 and D2/D3 agonists, increases γ oscillation power, and its effect is blocked by the highly specific D4R antagonist L-745,870. Using double in situ hybridization and immunofluorescence histochemistry, we show that hippocampal D4R mRNA and protein are more highly expressed in GAD67-positive GABAergic interneurons, many of which express the NRG-1 receptor ErbB4. Importantly, D4 and ErbB4 receptors are coexpressed in parvalbumin-positive basket cells that are critical for γ oscillations. Last, we report that D4R activation is essential for the effects of NRG-1 on network activity because L-745,870 and the atypical antipsychotic clozapine dramatically reduce the NRG-1-induced increase in γ oscillation power. This unique link between D4R and ErbB4 signaling on γ oscillation power, and their coexpression in parvalbumin-expressing interneurons, suggests a cellular mechanism that may be compromised in different psychiatric disorders affecting cognitive control. These findings are important given the association of a DRD4 polymorphism with alterations in attention, working memory, and γ oscillations, and suggest potential benefits of D4R modulators for targeting cognitive deficits.

5-Hydroxytryptamine1A receptor-activation hyperpolarizes pyramidal cells and suppresses hippocampal gamma oscillations via Kir3 channel activation.

Rhythmic cortical neuronal oscillations in the gamma frequency band (30-80 Hz, gamma oscillations) have been associated with cognitive processes such as sensory perception and integration, attention, learning, and memory. Gamma oscillations are disrupted in disorders for which cognitive deficits are hallmark symptoms such as schizophrenia and Alzheimer's disease.In vitro, various neurotransmitters have been found to modulate gamma oscillations. Serotonin(5-HT) has long been known to be important for both behavioural and cognitive functions such as learning and memory. Multiple 5-HT receptor subtypes are expressed in the CA3 region of the hippocampus and high doses of 5-HT reduce the power of induced gamma oscillations.Hypothesizing that 5-HT may have cell- and receptor subtype-specific modulatory effects, we investigated the receptor subtypes, cell types and cellular mechanisms engaged by 5-HT in the modulation of gamma oscillations in mice and rats. We found that 5-HT decreases the power of kainate-induced hippocampal gamma oscillations in both species via the 5-HT1A receptor subtype. Whole-cell patch clamp recordings demonstrated that this decrease was caused by a hyperpolarization of CA3 pyramidal cells and a reduction of their firing frequency, but not by alteration of inhibitory neurotransmission. Finally, our results show that the effect on pyramidal cells is mediated via the G protein-coupled receptor inwardly rectifying potassium channel Kir3.Our findings suggest this novel cellular mechanism as a potential target for therapies that are aimed at alleviating cognitive decline by helping the brain to maintain or re-establish normal gamma oscillation levels in neuropsychiatric and neurodegenerative disorders.

Synthesis and evaluation of antineurotoxicity properties of an amyloid-ß peptide targeting ligand containing a triamino acid.

Peptide-like compounds containing an arginine have been shown to bind and stabilize the central helix of the Alzheimer's disease related amyloid-ß peptide (Aß) in an α-helical conformation, thereby delaying its aggregation into cytotoxic species. Here we study a novel Aß targeting ligand AEDabDab containing the triamino acid, N(γ)-(2-aminoethyl)-2,4-diaminobutanoic (AEDab) acid. The new AEDab triamino acid carries an extra positive charge in the side chain and is designed to be incorporated into a ligand AEDabDab where the AEDab replaces an arginine moiety in a previously developed ligand Pep1b. This is done in order to increase the Aß-ligand interaction, and molecular dynamics (MD) simulation of the stability of the Aß central helix in the presence of the AEDabDab ligand shows further stabilization of the helical conformation of Aß compared to the previously reported Pep1b as well as compared to the AEOrnDab ligand containing an N(δ)-(2-aminoethyl)-2,5-diaminopentanoic acid unit which has an additional methylene group. To evaluate the effect of the AEDabDab ligand on the Aß neurotoxicity the AEDab triamino acid building block is synthesized by reductive alkylation of N-protected-glycinal with α-amino-protected diaminobutanoic acid, and the Aß targeting ligand AEDabDab is prepared by solid-phase synthesis starting with attachment of glutarate to the Wang support. Replacement of the arginine residue by the AEDab triamino acid resulted in an improved capability of the ligand to prevent the Aß1-42 induced reduction of gamma (γ) oscillations in hippocampal slice preparation.

TrkB signaling in parvalbumin-positive interneurons is critical for gamma-band network synchronization in hippocampus.

Although brain-derived neurotrophic factor (BDNF) is known to regulate circuit development and synaptic plasticity, its exact role in neuronal network activity remains elusive. Using mutant mice (TrkB-PV(-/-)) in which the gene for the BDNF receptor, tyrosine kinase B receptor (trkB), has been specifically deleted in parvalbumin-expressing, fast-spiking GABAergic (PV+) interneurons, we show that TrkB is structurally and functionally important for the integrity of the hippocampal network. The amplitude of glutamatergic inputs to PV+ interneurons and the frequency of GABAergic inputs to excitatory pyramidal cells were reduced in the TrkB-PV(-/-) mice. Functionally, rhythmic network activity in the gamma-frequency band (30-80 Hz) was significantly decreased in hippocampal area CA1. This decrease was caused by a desynchronization and overall reduction in frequency of action potentials generated in PV+ interneurons of TrkB-PV(-/-) mice. Our results show that the integration of PV+ interneurons into the hippocampal microcircuit is impaired in TrkB-PV(-/-) mice, resulting in decreased rhythmic network activity in the gamma-frequency band.

Signal peptide peptidase-like 2b modulates the amyloidogenic pathway and exhibits an Aß-dependent expression in Alzheimer's disease.

Alzheimer's disease (AD) is a multifactorial disorder driven by abnormal amyloid ß-peptide (Aß) levels. In this study, we investigated the role of presenilin-like signal peptide peptidase-like 2b (SPPL2b) in AD pathophysiology and its potential as a druggable target within the Aß cascade. Exogenous Aß42 influenced SPPL2b expression in human cell lines and acute mouse brain slices. SPPL2b and its AD-related substrate BRI2 were evaluated in the brains of AppNL-G-F knock-in AD mice and human postmortem AD brains. An early high cortical expression of SPPL2b was observed, followed by a downregulation in late AD pathology in AppNL-G-F mice, correlating with synaptic loss. To understand the consequences of pathophysiological SPPL2b dysregulation, we found that SPPL2b overexpression significantly increased APP cleavage, while genetic deletion reduced APP cleavage and Aß production. Notably, postmortem AD brains showed higher levels of SPPL2b's BRI2 substrate compared to healthy control samples. These results strongly support the involvement of SPPL2b in AD pathology. The early Aß-induced upregulation of SPPL2b may enhance Aß production in a vicious cycle, further aggravating Aß pathology. Therefore, SPPL2b emerges as a potential anti-Aß drug target.

Impaired astrocytic synaptic function by peripheral cholesterol metabolite 27-hydroxycholesterol.

Astrocytes represent the most abundant cell type in the brain, where they play critical roles in synaptic transmission, cognition, and behavior. Recent discoveries show astrocytes are involved in synaptic dysfunction during Alzheimer's disease (AD). AD patients have imbalanced cholesterol metabolism, demonstrated by high levels of side-chain oxidized cholesterol known as 27-hydroxycholesterol (27-OH). Evidence from our laboratory has shown that elevated 27-OH can abolish synaptic connectivity during neuromaturation, but its effect on astrocyte function is currently unclear. Our results suggest that elevated 27-OH decreases the astrocyte function in vivo in Cyp27Tg, a mouse model of brain oxysterol imbalance. Here, we report a downregulation of glutamate transporters in the hippocampus of CYP27Tg mice together with increased GFAP. GLT-1 downregulation was also observed when WT mice were fed with high-cholesterol diets. To study the relationship between astrocytes and neurons, we have developed a 3D co-culture system that allows all the cell types from mice embryos to differentiate in vitro. We report that our 3D co-cultures reproduce the effects of 27-OH observed in 2D neurons and in vivo. Moreover, we found novel degenerative effects in astrocytes that do not appear in 2D cultures, together with the downregulation of glutamate transporters GLT-1 and GLAST. We propose that this transporter dysregulation leads to neuronal hyperexcitability and synaptic dysfunction based on the effects of 27-OH on astrocytes. Taken together, these results report a new mechanism linking oxysterol imbalance in the brain and synaptic dysfunction through effects on astrocyte function.

Low dose of levetiracetam counteracts amyloid ß-induced alterations of hippocampal gamma oscillations by restoring fast-spiking interneuron activity.

Alzheimer's disease (AD) is characterized at an early stage by memory alterations that worsen during the development of the disease. Several clinical trials in phase 3 have failed despite being able to counteract classical AD-related alterations, possibly because of the lack of recovery of the regular neuronal network activity essential for memory including low gamma oscillations (γ-Osc). Nowadays, Levetiracetam (LEV), an SV2A modulator approved for epilepsy, is being used in trials with AD patients without further support for neurophysiological relevant effects on restoring the normal function of hippocampal neuronal network activity. Using concomitant recordings of local field potential γ-Osc and patch-clamp recordings of fast-spiking interneurons (FS-IN) on hippocampal slices of WT and AppNL-G-F AD animals, we found that LEV restores the power and rhythmicity of γ-Osc previously reduced by acute application of amyloid-ß on WT hippocampal slices, this effect is accompanied by the recovery of the synchronicity in the firing of FS-IN. In addition, we found that LEV counteracts the hippocampal γ-Osc alterations in the early prodromal stage of the disease in AppNL-G-F mice by recovering the rhythmicity of γ-Osc and the synchronicity in the firing of FS-IN. Altogether the results show that the precise modulation of neuronal circuits with LEV is a promising strategy to counteract early-stage alterations in hippocampal activity by modulating FS-IN in a memory-relevant neuronal network state like γ-Osc.

Targeting galectin-3 to counteract spike-phase uncoupling of fast-spiking interneurons to gamma oscillations in Alzheimer's disease.

BACKGROUND: Alzheimer's disease (AD) is a progressive multifaceted neurodegenerative disorder for which no disease-modifying treatment exists. Neuroinflammation is central to the pathology progression, with evidence suggesting that microglia-released galectin-3 (gal3) plays a pivotal role by amplifying neuroinflammation in AD. However, the possible involvement of gal3 in the disruption of neuronal network oscillations typical of AD remains unknown. METHODS: Here, we investigated the functional implications of gal3 signaling on experimentally induced gamma oscillations ex vivo (20-80 Hz) by performing electrophysiological recordings in the hippocampal CA3 area of wild-type (WT) mice and of the 5×FAD mouse model of AD. In addition, the recorded slices from WT mice under acute gal3 application were analyzed with RT-qPCR to detect expression of some neuroinflammation-related genes, and amyloid-ß (Aß) plaque load was quantified by immunostaining in the CA3 area of 6-month-old 5×FAD mice with or without Gal3 knockout (KO). RESULTS: Gal3 application decreased gamma oscillation power and rhythmicity in an activity-dependent manner, which was accompanied by impairment of cellular dynamics in fast-spiking interneurons (FSNs) and pyramidal cells. We found that the gal3-induced disruption was mediated by the gal3 carbohydrate-recognition domain and prevented by the gal3 inhibitor TD139, which also prevented Aß42-induced degradation of gamma oscillations. Furthermore, the 5×FAD mice lacking gal3 (5×FAD-Gal3KO) exhibited WT-like gamma network dynamics and decreased Aß plaque load. CONCLUSIONS: We report for the first time that gal3 impairs neuronal network dynamics by spike-phase uncoupling of FSNs, inducing a network performance collapse. Moreover, our findings suggest gal3 inhibition as a potential therapeutic strategy to counteract the neuronal network instability typical of AD and other neurological disorders encompassing neuroinflammation and cognitive decline.

Molecular Mechanisms of Amyloid-ß Self-Assembly Seeded by In Vivo-Derived Fibrils and Inhibitory Effects of the BRICHOS Chaperone.

Self-replication of amyloid-ß-peptide (Aß) fibril formation is a hallmark in Alzheimer's disease (AD). Detailed insights have been obtained in Aß self-assembly in vitro, yet whether similar mechanisms are relevant in vivo has remained elusive. Here, we investigated the ability of in vivo-derived Aß fibrils from two different amyloid precursor protein knock-in AD mouse models to seed Aß42 aggregation, where we quantified the microscopic rate constants. We found that the nucleation mechanism of in vivo-derived fibril-seeded Aß42 aggregation can be described with the same kinetic model as that in vitro. Further, we identified the inhibitory mechanism of the anti-amyloid BRICHOS chaperone on seeded Aß42 fibrillization, revealing a suppression of secondary nucleation and fibril elongation, which is strikingly similar as observed in vitro. These findings hence provide a molecular understanding of the Aß42 nucleation process triggered by in vivo-derived Aß42 propagons, providing a framework for the search for new AD therapeutics.