Amyloid-β Impairs Synaptic Inhibition via GABA(A) Receptor Endocytosis

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.

Seizures exacerbate excitatory: inhibitory imbalance in Alzheimer's disease and 5XFAD mice

Approximately 22% of Alzheimer's disease (AD) patients suffer from seizures, and the co-occurrence of seizures and epileptiform activity exacerbates AD pathology and related cognitive deficits, suggesting that seizures may be a targetable component of AD progression. Given that alterations in neuronal excitatory:inhibitory (E:I) balance occur in epilepsy, we hypothesized that decreased markers of inhibition relative to those of excitation would be present in AD patients. We similarly hypothesized that in 5XFAD mice, the E:I imbalance would progress from an early stage (prodromal) to later symptomatic stages and be further exacerbated by pentylenetetrazol (PTZ) kindling. Post-mortem AD temporal cortical tissues from patients with or without seizure history were examined for changes in several markers of E:I balance, including levels of the inhibitory GABAA receptor, the sodium potassium chloride cotransporter 1 (NKCC1) and potassium chloride cotransporter 2 (KCC2) and the excitatory NMDA and AMPA type glutamate receptors. We performed patch-clamp electrophysiological recordings from CA1 neurons in hippocampal slices and examined the same markers of E:I balance in prodromal 5XFAD mice. We next examined 5XFAD mice at chronic stages, after PTZ or control protocols, and in response to chronic mTORC1 inhibitor rapamycin, administered following kindled seizures, for markers of E:I balance. We found that AD patients with comorbid seizures had worsened cognitive and functional scores and decreased GABAA receptor subunit expression, as well as increased NKCC1/KCC2 ratios, indicative of depolarizing GABA responses. Patch clamp recordings of prodromal 5XFAD CA1 neurons showed increased intrinsic excitability, along with decreased GABAergic inhibitory transmission and altered glutamatergic neurotransmission, indicating that E:I imbalance may occur in early disease stages. Furthermore, seizure induction in prodromal 5XFAD mice led to later dysregulation of NKCC1/KCC2 and a reduction in GluA2 AMPA glutamate receptor subunit expression, indicative of depolarizing GABA receptors and calcium permeable AMPA receptors. Finally, we found that chronic treatment with the mTORC1 inhibitor, rapamycin, at doses we have previously shown to attenuate seizure-induced amyloid-β pathology and cognitive deficits, could also reverse elevations of the NKCC1/KCC2 ratio in these mice. Our data demonstrate novel mechanisms of interaction between AD and epilepsy and indicate that targeting E:I balance, potentially with US Food and Drug Administration-approved mTOR inhibitors, hold therapeutic promise for AD patients with a seizure history.

Longitudinal changes in the functional connectivity of individuals at risk of Alzheimer's disease

First-degree relatives of Alzheimer's disease patients constitute a key population in the search for early markers. Our group identified functional connectivity differences between cognitively unimpaired individuals with and without a family history. In this unprecedented follow-up study, we examine whether family history is associated with a longitudinal increase in the functional connectivity of those regions. Moreover, this is the first work to correlate electrophysiological measures with plasma p-tau231 levels, a known pathology marker, to interpret the nature of the change. We evaluated 69 cognitively unimpaired individuals with a family history of Alzheimer's disease and 28 without, at two different time points, approximately 3 years apart, including resting state magnetoencephalography recordings and plasma p-tau231 determinations. Functional connectivity changes in both precunei and left anterior cingulate cortex in the high-alpha band were studied using non-parametric cluster-based permutation tests. Connectivity values were correlated with p-tau231 levels. Three clusters emerged in individuals with family history, exhibiting a longitudinal increase of connectivity. Notably, the clusters for both precunei bore a striking resemblance to those found in previous cross-sectional studies. The connectivity values at follow-up and the change in connectivity in the left precuneus cluster showed significant positive correlations with p-tau231. This study consolidates the use of electrophysiology, in combination with plasma biomarkers, to monitor healthy individuals at risk of Alzheimer's disease and emphasizes the value of combining noninvasive markers to understand the underlying mechanisms and track disease progression. This could facilitate the design of more effective intervention strategies and accurate progression assessment tools.

Native-state proteomics of Parvalbumin interneurons identifies unique molecular signatures and vulnerabilities to early Alzheimer's pathology

Dysfunction in fast-spiking parvalbumin interneurons (PV-INs) may represent an early pathophysiological perturbation in Alzheimer's Disease (AD). Defining early proteomic alterations in PV-INs can provide key biological and translationally-relevant insights. We used cell-type-specific in-vivo biotinylation of proteins (CIBOP) coupled with mass spectrometry to obtain native-state PV-IN proteomes. PV-IN proteomic signatures include high metabolic and translational activity, with over-representation of AD-risk and cognitive resilience-related proteins. In bulk proteomes, PV-IN proteins were associated with cognitive decline in humans, and with progressive neuropathology in humans and the 5xFAD mouse model of Aβ pathology. PV-IN CIBOP in early stages of Aβ pathology revealed signatures of increased mitochondria and metabolism, synaptic and cytoskeletal disruption and decreased mTOR signaling, not apparent in whole-brain proteomes. Furthermore, we demonstrated pre-synaptic defects in PV-to-excitatory neurotransmission, validating our proteomic findings. Overall, in this study we present native-state proteomes of PV-INs, revealing molecular insights into their unique roles in cognitive resiliency and AD pathogenesis.

A Multiscale Closed-Loop Neurotoxicity Model of Alzheimer's Disease Progression Explains Functional Connectivity Alterations

The accumulation of amyloid-β (Aβ) and hyperphosphorylated-tau (hp-tau) are two classical histopathological biomarkers in Alzheimer's disease (AD). However, their detailed interactions with the electrophysiological changes at the meso- and macroscale are not yet fully understood. We developed a mechanistic multiscale model of AD progression, linking proteinopathy to its effects on neural activity and vice-versa. We integrated a heterodimer model of prion-like protein propagation and a brain network model of Jansen-Rit neural masses derived from human neuroimaging data whose parameters varied due to neurotoxicity. Results showed that changes in inhibition guided the electrophysiological alterations found in AD, and these changes were mainly attributed to Aβ effects. Additionally, we found a causal disconnection between cellular hyperactivity and interregional hypersynchrony contrary to previous beliefs. Finally, we demonstrated that early Aβ and hp-tau depositions' location determine the spatiotemporal profile of the proteinopathy. The presented model combines the molecular effects of both Aβ and hp-tau together with a mechanistic protein propagation model and network effects within a closed-loop model. This holds the potential to enlighten the interplay between AD mechanisms on various scales, aiming to develop and test novel hypotheses on the contribution of different AD-related variables to the disease evolution.

Changes in the Glutamate/GABA System in the Hippocampus of Rats with Age and during Alzheimer's Disease Signs Development

GABA and glutamate are the most abundant neurotransmitters in the CNS and play a pivotal part in synaptic stability/plasticity. Glutamate and GABA homeostasis is important for healthy aging and reducing the risk of various neurological diseases, while long-term imbalance can contribute to the development of neurodegenerative disorders, including Alzheimer's disease (AD). Normalization of the homeostasis has been discussed as a promising strategy for prevention and/or treatment of AD, however, data on the changes in the GABAergic and glutamatergic systems with age, as well as on the dynamics of AD development, are limited. It is not clear whether imbalance of the excitatory/inhibitory systems is the cause or the consequence of the disease development. Here we analyzed the age-related alterations of the levels of glutamate, GABA, as well as enzymes that synthesize them (glutaminase, glutamine synthetase, GABA-T, and GAD67), transporters (GLAST, GLT-1, and GAT1), and relevant receptors (GluA1, NMDAR1, NMDA2B, and GABAAr1) in the whole hippocampus of the Wistar rats and of the senescence-accelerated OXYS rats, a model of the most common (> 95%) sporadic AD. Our results suggest that there is a decline in glutamate and GABA signaling with age in hippocampus of the both rat strains. However, we have not identified significant changes or compensatory enhancements in this system in the hippocampus of OXYS rats during the development of neurodegenerative processes that are characteristic of AD.

Factors affecting the GABAergic synapse function in Alzheimer's disease: Focus on microRNAs

Alzheimer's disease (AD) is a progressive neurological disease characterized by the loss of cognitive function, confusion, and memory deficit. Accumulation of abnormal proteins, amyloid beta (Aß), and phosphorylated Tau (p-tau) forms plaques and tangles that deteriorate synapse function, resulting in neurodegeneration and cognitive decline in AD. The human brain is composed of different types of neurons and/or synapses that are functionally defective in AD. The GABAergic synapse, the most abundant inhibitory neuron in the human brain was found to be dysfunctional in AD and contributes to disrupting neurological function. This study explored the types of GABA receptors associated with neurological dysfunction and various biological and environmental factors that cause GABAergic neuron dysfunction in AD, such as Aβ, p-tau, aging, sex, astrocytes, microglia, APOE, mental disorder, diet, physical activity, and sleep. Furthermore, we explored the role of microRNAs (miRNAs) in the regulation of GABAergic synapse function in neurological disorders and AD states. We also discuss the molecular mechanisms underlying GABAergic synapse dysfunction with a focus on miR-27b, miR-30a, miR-190a/b, miR-33, miR-51, miR-129-5p, miR-376-3p, miR-376c, miR-30b and miR-502-3p. The purpose of our article is to highlight the recent research on miRNAs affecting the regulation of GABAergic synapse function and factors that contribute to the progression of AD.

Treadmill exercise prevents the hyperexcitability of pyramidal neurons in medial entorhinal cortex in the 3xTg-AD mouse model of Alzheimer's disease

Neuronal hyperactivity is a key abnormality in early stage Alzheimer's disease (AD). Medial entorhinal cortex (mEC) plays a vital role in memory function and is affected early in AD. Growing evidence indicates benefits of regular exercise on memory and cognitive function in humans with AD, although, the underlying mechanisms are not clear. Therefore, this study was designed to test the effects of 16 weeks treadmill exercise on spatial learning memory and the underlying cellular mechanisms in 6-month-old 3xTg-AD mice. Whole-cell patch clamp was used to examine neuronal intrinsic excitability, spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs) of mEC layer II/III pyramidal neurons in the following groups: wild type (WT + sham), 3xTg-AD (AD+sham), WT receiving exercise (WT + Ex), and AD receiving exercise (AD+Ex). We found that at a behavioral level, treadmill exercise decreased working memory errors in radial arm maze (RAM) test in 6-month-old AD mice. At a cellular level, we found that treadmill exercise prevented the abnormal increase in mEC pyramidal neuron input resistance and action potential firing in 6-month-old 3xTg-AD mice compared with WT + sham and AD+Ex mice; further, sEPSC amplitude and frequency were normal in AD+Ex but overactive in AD+sham; additionally, GABAergic inhibition was normal in AD+Ex mice but reduced in AD+sham. In conclusion, our results indicate that treadmill exercise improves spatial learning memory and prevents network hyperexcitability in mEC by reducing pyramidal neuronal intrinsic excitability and normalizing excitatory and inhibitory synaptic transmission in 3xTg-AD mice.

The intracerebral injection of Aβ1-42 oligomers does not invariably alter seizure susceptibility in mice

Objectives

Epileptiform activity and seizures are present in patients with Alzheimer's disease (AD) and genetic animal models of AD. Amyloid beta 1-42 (Aβ1-42) oligomers are thought to be crucial in AD and can cause neuronal hyperexcitability in vitro. However, it is unclear whether these Aβ1-42 oligomers cause the increased seizure susceptibility in vivo in people with AD and in AD animal models, nor via which mechanisms it would do so. We investigated this question by injecting Aβ1-42 oligomers intracerebrally in mice and assessed its impact on seizure susceptibility.

Materials and methods

We performed a single intracerebral injection of synthetic Aβ1-42 oligomers or scrambled Aβ1-42 in NMRI mice in three different cohorts and subjected them to an i.v. infusion of a chemoconvulsant. We evoked the seizures 1.5 h, 1 week, or 3 weeks after the intracerebral injection of Aβ1-42 oligomers, covering also the timepoints and injection locations that were used by others in similar experimental set-ups.

Results

With a thioflavine T assay and transmission electron microscopy we confirmed that Aβ1-42 monomers spontaneously aggregated to oligomers. We did not find an effect of Aβ1-42 oligomers on susceptibility to seizures - evoked 1.5 h, 1 week or 3 weeks - after their intracerebral injection.

Significance

The lack of effect of Aβ1-42 oligomers on seizure susceptibility in our experiments contrasts with recent findings in similar experimental set-ups. Contradicting conclusions are frequent in experiments with Aβ1-42 and they are often attributed to subtle differences in the various aggregation forms of the Aβ1-42 used in different experiments. We confirmed the presence of Aβ1-42 oligomers with state-of-the-art methods but cannot ascertain that the protein aggregates we used are identical to those used by others. Whether our findings or those previously published best represent the role of Aβ1-42 oligomers on seizures in AD remains unclear.

GABA decrease is associated with degraded neural specificity in the visual cortex of glaucoma patients

Glaucoma is an age-related neurodegenerative disease of the visual system, affecting both the eye and the brain. Yet its underlying metabolic mechanisms and neurobehavioral relevance remain largely unclear. Here, using proton magnetic resonance spectroscopy and functional magnetic resonance imaging, we investigated the GABAergic and glutamatergic systems in the visual cortex of glaucoma patients, as well as neural specificity, which is shaped by GABA and glutamate signals and underlies efficient sensory and cognitive functions. Our study shows that among the older adults, both GABA and glutamate levels decrease with increasing glaucoma severity regardless of age. Further, our study shows that the reduction of GABA but not glutamate predicts the neural specificity. This association is independent of the impairments on the retina structure, age, and the gray matter volume of the visual cortex. Our results suggest that glaucoma-specific decline of GABA undermines neural specificity in the visual cortex and that targeting GABA could improve the neural specificity in glaucoma.

Native-state proteomics of Parvalbumin interneurons identifies novel molecular signatures and metabolic vulnerabilities to early Alzheimer's disease pathology

One of the earliest pathophysiological perturbations in Alzheimer's Disease (AD) may arise from dysfunction of fast-spiking parvalbumin (PV) interneurons (PV-INs). Defining early protein-level (proteomic) alterations in PV-INs can provide key biological and translationally relevant insights. Here, we use cell-type-specific in vivo biotinylation of proteins (CIBOP) coupled with mass spectrometry to obtain native-state proteomes of PV interneurons. PV-INs exhibited proteomic signatures of high metabolic, mitochondrial, and translational activity, with over-representation of causally linked AD genetic risk factors. Analyses of bulk brain proteomes indicated strong correlations between PV-IN proteins with cognitive decline in humans, and with progressive neuropathology in humans and mouse models of Aβ pathology. Furthermore, PV-IN-specific proteomes revealed unique signatures of increased mitochondrial and metabolic proteins, but decreased synaptic and mTOR signaling proteins in response to early Aβ pathology. PV-specific changes were not apparent in whole-brain proteomes. These findings showcase the first native state PV-IN proteomes in mammalian brain, revealing a molecular basis for their unique vulnerabilities in AD.

Discoveries and future significance of research into amyloid-beta/α7-containing nicotinic acetylcholine receptor (nAChR) interactions

Initiated by findings that Alzheimer's disease is associated with a profound loss of cholinergic markers in human brain, decades of studies have examined the interactions between specific subtypes of nicotinic acetylcholine receptors and amyloid-β [derive from the amyloid precursor protein (APP), which is cleaved to yield variable isoforms of amyloid-β]. We review the evolving understanding of amyloid-β's roles in Alzheimer's disease and pioneering studies that highlighted a role of nicotinic acetylcholine receptors in mediating important aspects of amyloid-β's effects. This review also surveys the current state of research into amyloid-β / nicotinic acetylcholine receptor interactions. The field has reached an exciting point in which common themes are emerging from the wide range of prior research and a range of accessible, relevant model systems are available to drive further progress. We highlight exciting new areas of inquiry and persistent challenges that need to be considered while conducting this research. Studies of amyloid-β and nicotinic acetylcholine receptor populations that it interacts with provide opportunities for innovative basic and translational scientific breakthroughs related to nicotinic receptors biology, Alzheimer's disease, and cholinergic contributions to cognition more broadly.

Exploring the Pathological Effect of Aβ42 Oligomers on Neural Networks in Primary Cortical Neuron Culture

Alzheimer’s disease (AD) is a multifactorial disorder that affects cognitive functioning, behavior, and neuronal properties. The neuronal dysfunction is primarily responsible for cognitive decline in AD patients, with many causal factors including plaque accumulation of Aβ42. Neural hyperactivity induced by Aβ42 deposition causes abnormalities in neural networks, leading to alterations in synaptic activity and interneuron dysfunction. Even though neuroimaging techniques elucidated the underlying mechanism of neural connectivity, precise understanding at the cellular level is still elusive. Previous multielectrode array studies have examined the neuronal network modulation in in vitro cultures revealing the relevance of ion channels and the chemical modulators in the presence of Aβ42. In this study, we investigated neuronal connectivity and dynamic changes using a high-density multielectrode array, particularly looking at network-wide parameter changes over time. By comparing the neuronal network between normal and Aβ42treated neuronal cultures, it was possible to discover the direct pathological effect of the Aβ42 oligomer altering the network characteristics. The detrimental effects of the Aβ42 oligomer included not only a decline in spike activation but also a qualitative impairment in neural connectivity as well as a disorientation of dispersibility. As a result, this will improve our understanding of how neural networks are modified during AD progression.

Pinpointing the locus of GABAergic vulnerability in Alzheimer's disease

The early stages of Alzheimer's disease (AD) have been linked to microcircuit dysfunction and pathophysiological neuronal firing in several brain regions. Inhibitory GABAergic microcircuitry is a critical feature of stable neural-circuit function in the healthy brain, and its dysregulation has therefore been proposed as contributing to AD-related pathophysiology. However, exactly how the critical balance between excitatory and inhibitory microcircuitry is modified by AD pathogenesis remains unclear. Here, we set the current evidence implicating dysfunctional GABAergic microcircuitry as a driver of early AD pathophysiology in a simple conceptual framework. Our framework is based on a generalised reductionist model of firing-rate control by local feedback inhibition. We use this framework to consider multiple loci that may be vulnerable to disruption by AD pathogenesis. We first start with evidence investigating how AD-related processes may impact the gross number of inhibitory neurons in the network. We then move to discuss how pathology may impact intrinsic cellular properties and firing thresholds of GABAergic neurons. Finally, we cover how AD-related pathogenesis may disrupt synaptic connectivity between excitatory and inhibitory neurons. We use the feedback inhibition framework to discuss and organise the available evidence from both preclinical rodent work and human studies in AD patients and conclude by identifying key questions and understudied areas for future investigation.

Excitation-inhibition imbalance disrupts visual familiarity in amyloid and non-pathology conditions

Neuronal hyperactivity induces memory deficits in Alzheimer's disease. However, how hyperactivity disrupts memory is unclear. Using in vivo synaptic imaging in the mouse visual cortex, we show that structural excitatory-inhibitory synapse imbalance in the apical dendrites favors hyperactivity in early amyloidosis. Consistent with this, natural images elicit neuronal hyperactivity in these mice. Compensatory changes that maintain activity homeostasis disrupt functional connectivity and increase population sparseness such that a small fraction of neurons dominates population activity. These properties reduce the selectivity of neural response to natural images and render visual recognition memory vulnerable to interference. Deprivation of non-specific visual experiences improves the neural representation and behavioral expression of visual familiarity. In contrast, in non-pathological conditions, deprivation of non-specific visual experiences induces disinhibition, increases excitability, and disrupts visual familiarity. We show that disrupted familiarity occurs when the fraction of high-responsive neurons and the persistence of neural representation of a memory-associated stimulus are not constrained.

The Anaesthetics Isoflurane and Xenon Reverse the Synaptotoxic Effects of Aβ1-42 on Megf10-Dependent Astrocytic Synapse Elimination and Spine Density in Ex Vivo Hippocampal Brain Slices

It has been hypothesised that inhalational anaesthetics such as isoflurane (Iso) may trigger the pathogenesis of Alzheimer's disease (AD), while the gaseous anaesthetic xenon (Xe) exhibits many features of a putative neuroprotective agent. Loss of synapses is regarded as one key cause of dementia in AD. Multiple EGF-like domains 10 (MEGF10) is one of the phagocytic receptors which assists the elimination of synapses by astrocytes. Here, we investigated how β-amyloid peptide 1-42 (Aβ_1-42), Iso and Xe interact with MEGF10-dependent synapse elimination. Murine cultured astrocytes as well as cortical and hippocampal ex vivo brain slices were treated with either Aβ_1-42, Iso or Xe and the combination of Aβ_1-42 with either Iso or Xe. We quantified MEGF10 expression in astrocytes and dendritic spine density (DSD) in slices. In brain slices of wild type and AAV-induced MEGF10 knock-down mice, antibodies against astrocytes (GFAP), pre- (synaptophysin) and postsynaptic (PSD95) components were used for co-localization analyses by means of immunofluorescence-imaging and 3D rendering techniques. Aβ_1-42 elevated pre- and postsynaptic components inside astrocytes and decreased DSD. The combined application with either Iso or Xe reversed these effects. In the presence of Aβ_1-42 both anaesthetics decreased MEGF10 expression. AAV-induced knock-down of MEGF10 reduced the pre- and postsynaptic marker inside astrocytes. The presented data suggest Iso and Xe are able to reverse the Aβ_1-42-induced enhancement of synaptic elimination in ex vivo hippocampal brain slices, presumably through MEGF10 downregulation.

Modifiable risk factors of dementia linked to excitation-inhibition imbalance

Recent evidence identifies 12 potentially modifiable risk factors for dementia to which 40% of dementia cases are attributed. While the recognition of these risk factors has paved the way for the development of new prevention measures, the link between these risk factors and the underlying pathophysiology of dementia is yet not well understood. A growing number of recent clinical and preclinical studies support a role of Excitation-Inhibition (E-I) imbalance in the pathophysiology of dementia. In this review, we aim to propose a conceptual model on the links between the modifiable risk factors and the E-I imbalance in dementia. This model, which aims to address the current gap in the literature, is based on 12 mediating common mechanisms: the hypothalamic-pituitary-adrenal (HPA) axis dysfunction, neuroinflammation, oxidative stress, mitochondrial dysfunction, cerebral hypo-perfusion, blood-brain barrier (BBB) dysfunction, beta-amyloid deposition, elevated homocysteine level, impaired neurogenesis, tau tangles, GABAergic dysfunction, and glutamatergic dysfunction. We believe this model serves as a framework for future studies in this field and facilitates future research on dementia prevention, discovery of new biomarkers, and developing new interventions.

A multiscale brain network model links Alzheimer’s disease-mediated neuronal hyperactivity to large-scale oscillatory slowing

Background

Neuronal hyperexcitability and inhibitory interneuron dysfunction are frequently observed in preclinical animal models of Alzheimer’s disease (AD). This study investigates whether these microscale abnormalities explain characteristic large-scale magnetoencephalography (MEG) activity in human early-stage AD patients.

Methods

To simulate spontaneous electrophysiological activity, we used a whole-brain computational network model comprised of 78 neural masses coupled according to human structural brain topology. We modified relevant model parameters to simulate six literature-based cellular scenarios of AD and compare them to one healthy and six contrast (non-AD-like) scenarios. The parameters include excitability, postsynaptic potentials, and coupling strength of excitatory and inhibitory neuronal populations. Whole-brain spike density and spectral power analyses of the simulated data reveal mechanisms of neuronal hyperactivity that lead to oscillatory changes similar to those observed in MEG data of 18 human prodromal AD patients compared to 18 age-matched subjects with subjective cognitive decline.

Results

All but one of the AD-like scenarios showed higher spike density levels, and all but one of these scenarios had a lower peak frequency, higher spectral power in slower (theta, 4–8Hz) frequencies, and greater total power. Non-AD-like scenarios showed opposite patterns mainly, including reduced spike density and faster oscillatory activity. Human AD patients showed oscillatory slowing (i.e., higher relative power in the theta band mainly), a trend for lower peak frequency and higher total power compared to controls. Combining model and human data, the findings indicate that neuronal hyperactivity can lead to oscillatory slowing, likely due to hyperexcitation (by hyperexcitability of pyramidal neurons or greater long-range excitatory coupling) and/or disinhibition (by reduced excitability of inhibitory interneurons or weaker local inhibitory coupling strength) in early AD.

Conclusions

Using a computational brain network model, we link findings from different scales and models and support the hypothesis of early-stage neuronal hyperactivity underlying E/I imbalance and whole-brain network dysfunction in prodromal AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-01041-4.

Cortical hyperexcitability and plasticity in Alzheimer's disease: developments in understanding and management

INTRODUCTION

Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiological tool that provides important insights into Alzheimer's Disease (AD). A significant body of work utilizing TMS techniques has explored the pathophysiological relevance of cortical hyperexcitability and plasticity in AD and their modulation in novel therapies.

AREAS COVERED

This review examines the technique of TMS, the use of TMS to examine specific features of cortical excitability and the use of TMS techniques to modulate cortical function. A search was performed utilizing the PubMed database to identify key studies utilizing TMS to examine cortical hyperexcitability and plasticity in Alzheimer's dementia. We then translate this understanding to the study of Alzheimer's disease pathophysiology, examining the underlying neurophysiologic links contributing to these twin signatures, cortical hyperexcitability and abnormal plasticity, in the cortical dysfunction characterizing AD. Finally, we examine utilization of TMS excitability to guide targeted therapies and, through the use of repetitive TMS (rTMS), modulate cortical plasticity.

EXPERT OPINION

The examination of cortical hyperexcitability and plasticity with TMS has potential to optimize and expand the window of therapeutic interventions in AD, though remains at relatively early stage of development.

Molecular Mechanisms of Epilepsy: The Role of the Chloride Transporter KCC2

Epilepsy is a neurological disease characterized by abnormal or synchronous brain activity causing seizures, which may produce convulsions, minor physical signs, or a combination of symptoms. These disorders affect approximately 65 million people worldwide, from all ages and genders. Seizures apart, epileptic patients present a high risk to develop neuropsychological comorbidities such as cognitive deficits, emotional disturbance, and psychiatric disorders, which severely impair quality of life. Currently, the treatment for epilepsy includes the administration of drugs or surgery, but about 30% of the patients treated with antiepileptic drugs develop time-dependent pharmacoresistence. Therefore, further investigation about epilepsy and its causes is needed to find new pharmacological targets and innovative therapeutic strategies. Pharmacoresistance is associated to changes in neuronal plasticity and alterations of GABA_A receptor-mediated neurotransmission. The downregulation of GABA inhibitory activity may arise from a positive shift in GABA_A receptor reversal potential, due to an alteration in chloride homeostasis. In this paper, we review the contribution of K⁺-Cl^--cotransporter (KCC2) to the alterations in the Cl^- gradient observed in epileptic condition, and how these alterations are coupled to the increase in the excitability.

Beta-Site Amyloid Precursor Protein-Cleaving Enzyme Inhibition Partly Restores Sevoflurane-Induced Deficits on Synaptic Plasticity and Spine Loss

Evidence indicates that inhalative anesthetics enhance the β-site amyloid precursor protein (APP)-cleaving enzyme (BACE) activity, increase amyloid beta 1-42 (Aβ_1–42) aggregation, and modulate dendritic spine dynamics. However, the mechanisms of inhalative anesthetics on hippocampal dendritic spine plasticity and BACE-dependent APP processing remain unclear. In this study, hippocampal slices were incubated with equipotent isoflurane (iso), sevoflurane (sevo), or xenon (Xe) with/without pretreatment of the BACE inhibitor LY2886721 (LY). Thereafter, CA1 dendritic spine density, APP processing-related molecule expressions, nectin-3 levels, and long-term potentiation (LTP) were tested. The nectin-3 downregulation on LTP and dendritic spines were evaluated. Sevo treatment increased hippocampal mouse Aβ_1–42 (mAβ_1–42), abolished CA1-LTP, and decreased spine density and nectin-3 expressions in the CA1 region. Furthermore, CA1-nectin-3 knockdown blocked LTP and reduced spine density. Iso treatment decreased spine density and attenuated LTP. Although Xe blocked LTP, it did not affect spine density, mAβ_1–42, or nectin-3. Finally, antagonizing BACE activity partly restored sevo-induced deficits. Taken together, our study suggests that sevo partly elevates BACE activity and interferes with synaptic remodeling, whereas iso mildly modulates synaptic changes in the CA1 region of the hippocampus. On the other hand, Xe does not alternate dendritic spine remodeling.

Cognitive Training Modulates Brain Hypersynchrony in a Population at Risk for Alzheimer's Disease

BACKGROUND

Recent studies demonstrated that brain hypersynchrony is an early sign of dysfunction in Alzheimer's disease (AD) that can represent a proxy for clinical progression. Conversely, non-pharmacological interventions, such as cognitive training (COGTR), are associated with cognitive gains that may be underpinned by a neuroprotective effect on brain synchrony.

OBJECTIVE

To study the potential of COGTR to modulate brain synchrony and to eventually revert the hypersynchrony phenomenon that characterizes preclinical AD.

METHODS

The effect of COGTR was examined in a sample of healthy controls (HC, n = 41, 22 trained) and individuals with subjective cognitive decline (SCD, n = 49, 24 trained). Magnetoencephalographic (MEG) activity and neuropsychological scores were acquired before and after a ten-week COGTR intervention aimed at improving cognitive function and daily living performance. Functional connectivity (FC) was analyzed using the phase-locking value. A mixed-effects ANOVA model with factors time (pre-intervention/post-intervention), training (trained/non-trained), and diagnosis (HC/SCD) was used to investigate significant changes in FC.

RESULTS

We found an average increase in alpha-band FC over time, but the effect was different in each group (trained and non-trained). In the trained group (HC and SCD), we report a reduction in the increase in FC within temporo-parietal and temporo-occipital connections. In the trained SCD group, this reduction was stronger and showed a tentative correlation with improved performance in different cognitive tests.

CONCLUSION

COGTR interventions could mitigate aberrant increases in FC in preclinical AD, promoting brain synchrony normalization in groups at a higher risk of developing dementia.

Suppression of hippocampal GABAergic transmission impairs memory in rodent models of Alzheimer's disease

Emerging evidence demonstrates the potential involvement of hippocampal GABAergic transmission in the process of memory acquisition and consolidation, while no consistent report is available to address the adaptation of hippocampal GABAergic transmission and its contribution to memory deficiency in the setting of Alzheimer's disease (AD). Brain-derived neurotrophic factor (BDNF) is a key molecule that regulates GABAergic transmission. In the brain, mature BDNF is generated from the proteolytic cleavage of proBDNF, while BDNF and proBDNF have differential effects on central GABAergic transmission. First, the present study reports a remarkable increase of proBDNF/BNDF ratio in the hippocampal CA1 area in rodent models of AD, indicating a potential impaired process of BDNF maturation from proBDNF cleavage. We report a suppressed hippocampal GABAergic strength, potentially resulting from the reduced expression of anion chloride co-transporter KCC2 and subsequent positive shift of GABAergic Cl-equilibrium potential (E_Cl-), which is attenuated by microinjection of BDNF with proBDNF inhibitor TAT-Pep5. We also show that normalization of proBDNF/BDNF signaling or GABAergic E_Cl-by intracerebroventricular (i.c.v.) administration of bumetanide remarkably improves the cognitive performance in Morris water maze test and fear conditioning test in rodent models of AD. These results demonstrate a critical role of hippocampal proBDNF/BDNF in regulating GABAergic transmission and contributing to memory dysfunction in rodent models of AD.

Impaired Experience-Dependent Refinement of Place Cells in a Rat Model of Alzheimer's Disease

BACKGROUND

Hippocampal place cells play an integral role in generating spatial maps. Impaired spatial memory is a characteristic pathology of Alzheimer's disease (AD), yet it remains unclear how AD influences the properties of hippocampal place cells.

OBJECTIVE

To record electrophysiological activity in hippocampal CA1 neurons in freely-moving 18-month-old male TgF344-AD and age-matched wild-type (WT) littermates to examine place cell properties.

METHODS

We implanted 32-channel electrode arrays into the CA1 subfield of 18-month-old male WT and TgF344-AD (n = 6/group) rats. Ten days after implantation, single unit activity in an open field arena was recorded across days. The spatial information content, in-field firing rate, and stability of each place cell was compared across groups. Pathology was assessed by immunohistochemical staining, and a deep neural network approach was used to count cell profiles.

RESULTS

Aged TgF344-AD rats exhibited hippocampal amyloid-β deposition, and a significant increase in Iba1 immunoreactivity and microglia cell counts. Place cells from WT and TgF344-AD rat showed equivalent spatial information, in-field firing rates, and place field stability when initially exposed to the arena. However, by day 3, the place cells in aged WT rats showed characteristic spatial tuning as evidenced by higher spatial information content, stability, and in-field firing rates, an effect not seen in TgF344-AD rats.

CONCLUSION

These findings support the notion that altered electrophysiological properties of place cells may contribute to the learning and memory deficits observed in AD.

Gut feelings: the microbiota-gut-brain axis on steroids

The intricate connection between central and enteric nervous systems is well established with emerging evidence linking gut microbiota function as a significant new contributor to gut-brain axis signaling. Several microbial signals contribute to altered gut-brain communications, with steroids representing an important biological class that impacts central and enteric nervous system function. Neuroactive steroids contribute pathologically to neurological disorders, including dementia and depression, by modulating the activity of neuroreceptors. However, limited information is available on the influence of neuroactive steroids on the enteric nervous system and gastrointestinal function. In this review, we outline how steroids can modulate enteric nervous system function by focusing on their influence on different receptors that are present in the intestine in health and disease. We also highlight the potential role of the gut microbiota in modulating neuroactive steroid signaling along the gut-brain axis.

Significance of GABAA Receptor for Cognitive Function and Hippocampal Pathology

The hippocampus is a primary area for contextual memory, known to process spatiotemporal information within a specific episode. Long-term strengthening of glutamatergic transmission is a mechanism of contextual learning in the dorsal cornu ammonis 1 (CA1) area of the hippocampus. CA1-specific immobilization or blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor delivery can impair learning performance, indicating a causal relationship between learning and receptor delivery into the synapse. Moreover, contextual learning also strengthens GABA_A (gamma-aminobutyric acid) receptor-mediated inhibitory synapses onto CA1 neurons. Recently we revealed that strengthening of GABA_A receptor-mediated inhibitory synapses preceded excitatory synaptic plasticity after contextual learning, resulting in a reduced synaptic excitatory/inhibitory (E/I) input balance that returned to pretraining levels within 10 min. The faster plasticity at inhibitory synapses may allow encoding a contextual memory and prevent cognitive dysfunction in various hippocampal pathologies. In this review, we focus on the dynamic changes of GABA_A receptor mediated-synaptic currents after contextual learning and the intracellular mechanism underlying rapid inhibitory synaptic plasticity. In addition, we discuss that several pathologies, such as Alzheimer’s disease, autism spectrum disorders and epilepsy are characterized by alterations in GABA_A receptor trafficking, synaptic E/I imbalance and neuronal excitability.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

Neurotransmitter-stimulated neuron-derived sEVs have opposite effects on amyloid β-induced neuronal damage

The ratio of excitatory to inhibitory neurotransmitters is essential for maintaining the firing patterns of neural networks, and is strictly regulated within individual neurons and brain regions. Excitatory to inhibitory (E/I) imbalance has been shown to participate in the progression of neurodegenerative diseases, including Alzheimer's disease (AD). Glutamate excitotoxicity and GABAergic neuron dysfunction appear to be key components of the neuronal cell death that takes place in AD. Since extracellular vesicles (EVs) are now explored as an important vehicle in transmitting signals between cells, we hypothesized that the function of neuron-derived small EVs (sEVs) might be regulated by the status of neurotransmitter balance and that sEVs might affect amyloid β (Aβ) toxicity on neurons. This study aimed to reveal the effects of sEVs from unbalanced neurotransmitter-stimulated neurons on Aβ-induced toxicity. We demonstrated the opposite effects of the two groups of sEVs isolated from neurons stimulated by glutamate or GABA on Aβ toxicity in vivo and in vitro. The sEVs released from GABA-treated neurons alleviated Aβ-induced damage, while those released from glutamate-treated neurons aggravated Aβ toxicity. Furthermore, we compared the microRNA (miRNA) composition of sEVs isolated from glutamate/GABA/PBS-treated neurons. Our results showed that glutamate and GABA oppositely regulated miR-132 levels in sEVs, resulting in the opposite destiny of recipient cells challenged with Aβ. Our results indicated that manipulating the function of sEVs by different neurotransmitters may reveal the mechanisms underlying the pathogenesis of AD and provide a promising strategy for AD treatment.

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 (Aβ42). Failure of Aβ42 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 Aβ42 aggregation, can rescue the gamma rhythm. We demonstrate that Aβ42 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 Aβ42-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.

Neuronal excitation/inhibition imbalance: core element of a translational perspective on Alzheimer pathophysiology

Our incomplete understanding of the link between Alzheimer's Disease pathology and symptomatology is a crucial obstacle for therapeutic success. Recently, translational studies have begun to connect the dots between protein alterations and deposition, brain network dysfunction and cognitive deficits. Disturbance of neuronal activity, and in particular an imbalance in underlying excitation/inhibition (E/I), appears early in AD, and can be regarded as forming a central link between structural brain pathology and cognitive dysfunction. While there are emerging (non-)pharmacological options to influence this imbalance, the complexity of human brain dynamics has hindered identification of an optimal approach. We suggest that focusing on the integration of neurophysiological aspects of AD at the micro-, meso- and macroscale, with the support of computational network modeling, can unite fundamental and clinical knowledge, provide a general framework, and suggest rational therapeutic targets.

GABAergic dysfunction, neural network hyperactivity and memory impairments in human aging and Alzheimer's disease

In this review, we focus on the potential role of the γ-aminobutyric acidergic (GABAergic) system in age-related episodic memory impairments in humans, with a particular focus on Alzheimer's disease (AD). Well-established animal models have shown that GABA plays a central role in regulating and synchronizing neuronal signaling in the hippocampus, a brain area critical for episodic memory that undergoes early and significant morphologic and functional changes in the course of AD. Neuroimaging research in humans has documented hyperactivity in the hippocampus and losses of resting state functional connectivity in the Default Mode Network, a network that itself prominently includes the hippocampus-presaging episodic memory decline in individuals at-risk for AD. Apolipoprotein ε4, the highest genetic risk factor for AD, is associated with GABAergic dysfunction in animal models, and episodic memory impairments in humans. In combination, these findings suggest that GABA may be the linchpin in a complex system of factors that eventually leads to the principal clinical hallmark of AD: episodic memory loss. Here, we will review the current state of literature supporting this hypothesis. First, we will focus on the molecular and cellular basis of the GABAergic system and its role in memory and cognition. Next, we report the evidence of GABA dysregulations in AD and normal aging, both in animal models and human studies. Finally, we outline a model of GABAergic dysfunction based on the results of functional neuroimaging studies in humans, which have shown hippocampal hyperactivity to episodic memory tasks concurrent with and even preceding AD diagnosis, along with factors that may modulate this association.

Identification of intraneuronal amyloid beta oligomers in locus coeruleus neurons of Alzheimer's patients and their potential impact on inhibitory neurotransmitter receptors and neuronal excitability

AIMS

Amyloid β-oligomers (AβO) are potent modulators of Alzheimer's pathology, yet their impact on one of the earliest brain regions to exhibit signs of the condition, the locus coeruleus (LC), remains to be determined. Of particular importance is whether AβO impact the spontaneous excitability of LC neurons. This parameter determines brain-wide noradrenaline (NA) release, and thus NA-mediated brain functions, including cognition, emotion and immune function, which are all compromised in Alzheimer's patients. Therefore, the aim of the study was to determine the expression profile of AβO in the LC of Alzheimer's patients and to probe their potential impact on the molecular and functional correlates of LC excitability, using a mouse model of increased Aβ production (APP-PSEN1).

METHODS AND RESULTS

Immunohistochemistry and confocal microscopy, using AβO-specific antibodies, confirmed LC AβO expression both intraneuronally and extracellularly in both Alzheimer's and APP-PSEN1 samples. Patch clamp electrophysiology recordings revealed that APP-PSEN1 LC neuronal hyperexcitability accompanied this AβO expression profile, arising from a diminished inhibitory effect of GABA due to impaired expression and function of the GABA-A receptor (GABAA R) α3 subunit. This altered LC α3-GABAA R expression profile overlapped with AβO expression in samples from both APP-PSEN1 mice and Alzheimer's patients. Finally, strychnine-sensitive glycine receptors (GlyRs) remained resilient to Aβ-induced changes and their activation reversed LC hyperexcitability.

CONCLUSIONS

The data suggest a close association between AβO and α3-GABAA Rs in the LC of Alzheimer's patients, and their potential to dysregulate LC activity, thereby contributing to the spectrum of pathology of the LC-NA system in this condition.

Metabolic disorder in Alzheimer's disease

Alzheimer's disease (AD), a well known aging-induced neurodegenerative disease is related to amyloid proteinopathy. This proteinopathy occurs due to abnormalities in protein folding, structure and thereby its function in cells. The root cause of such kind of proteinopathy and its related neurodegeneration is a disorder in metabolism, rather metabolomics of the major as well as minor nutrients. Metabolomics is the most relevant "omics" platform that offers a great potential for the diagnosis and prognosis of neurodegenerative diseases as an individual's metabolome. In recent years, the research on such kinds of neurodegenerative diseases, especially aging-related disorders is broadened its scope towards metabolic function. Different neurotransmitter metabolisms are also involved with AD and its associated neurodegeneration. The genetic and epigenetic backgrounds are also noteworthy. In this review, the physiological changes of AD in relation to its corresponding biochemical, genetic and epigenetic involvements including its (AD) therapeutic aspects are discussed.

An Unbalanced Synaptic Transmission: Cause or Consequence of the Amyloid Oligomers Neurotoxicity?

Amyloid-β (Aβ) 1-40 and 1-42 peptides are key mediators of synaptic and cognitive dysfunction in Alzheimer's disease (AD). Whereas in AD, Aβ is found to act as a pro-epileptogenic factor even before plaque formation, amyloid pathology has been detected among patients with epilepsy with increased risk of developing AD. Among Aβ aggregated species, soluble oligomers are suggested to be responsible for most of Aβ's toxic effects. Aβ oligomers exert extracellular and intracellular toxicity through different mechanisms, including interaction with membrane receptors and the formation of ion-permeable channels in cellular membranes. These damages, linked to an unbalance between excitatory and inhibitory neurotransmission, often result in neuronal hyperexcitability and neural circuit dysfunction, which in turn increase Aβ deposition and facilitate neurodegeneration, resulting in an Aβ-driven vicious loop. In this review, we summarize the most representative literature on the effects that oligomeric Aβ induces on synaptic dysfunction and network disorganization.

Pinoresinol diglucoside attenuates neuroinflammation, apoptosis and oxidative stress in a mice model with Alzheimer's disease

For Alzheimer's disease (AD), there is still no effective treatment strategy. Pinoresinol diglucoside (PDG) is one of the major lignans isolated from Eucommia ulmoides. It is endowed with multiple pharmacological activities, including anti-inflammatory, antioxidant and anticancer activities. In this study, we investigated the potential neuroprotective functions of PDG in AD. Mice model with AD was established adopting stereotactic hippocampal injection of Aβ1-42 (410 pmol/mouse), and 3 days later, mice were administrated with 5 and 10 mg/kg PDG by intragastric administration every day for 3 weeks. Morris water maze and Y-maze tests demonstrated that PDG treatment could markedly reverse Aβ1-42-induced memory impairment in mice. It is found that PDG restrained the release of proinflammatory cytokines (tumor necrosis factor α and interleukin 1β), reactive oxygen species and malondialdehyde, and promoted the activity of the antioxidant enzyme (superoxide dismutase and catalase) by quantitative real-time-PCR, colorimetric method and ELISA assay. Western blot assay results have shown that PDG could also upregulate the ratio of Bcl-2/Bax and downregulate cytochrome c and cleaved caspase-3 expressions, thereby inhibiting neuronal apoptosis. Furthermore, PDG also significantly reduced the expression of Toll-like receptor 4 (TLR4) and the activation of nuclear factor-κB (NF-κB) p65, and promoted nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase 1 (HO-1) expressions. In conclusion, PDG can attenuate neuroinflammation, neuronal apoptosis and oxidative stress through the TLR4/NF-κB and Nrf2/HO-1 pathways, and ameliorate memory dysfunction induced by Aβ1-42 in mice.

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.

Transcriptome Changes in the Alzheimer's Disease Middle Temporal Gyrus: Importance of RNA Metabolism and Mitochondria-Associated Membrane Genes

We used Illumina Human HT-12 v4 arrays to compare RNA expression of middle temporal gyrus (MTG; BA21) in Alzheimer's disease (AD = 97) and non-demented controls (ND = 98). A total of 938 transcripts were highly differentially expressed (adj p < 0.01; log2 FC ≥ |0.500|, with 411 overexpressed and 527 underexpressed in AD. Our results correlated with expression profiling in neurons from AD and ND obtained by laser capture microscopy in MTG from an independent dataset (log2 FC correlation: r = 0.504; p = 2.2e-16). Additionally, selected effects were validated by qPCR. ANOVA analysis yielded no difference between genders in response to AD, but some gender specific genes were detected (e.g., IL8 and AGRN in males, and HSPH1 and GRM1 in females). Several transcripts were associated with Braak staging (e.g., AEBP1 and DNALI1), antemortem MMSE (e.g., AEBP1 and GFAP), and tangle density (e.g., RNU1G2, and DNALI1). At the pathway level, we detected enrichment of synaptic vesicle processes and GABAergic transmission genes. Finally, applying the Weighted Correlation Network Analysis, we identified four expression modules enriched for neuronal and synaptic genes, mitochondria-associated membrane, chemical stimulus and olfactory receptor and non-coding RNA metabolism genes. Our results represent an extensive description of MTG mRNA profiling in a large sample of AD and ND. These data provide a list of genes associated with AD, and correlated to neurofibrillary tangles density. In addition, these data emphasize the importance of mitochondrial membranes and transcripts related to olfactory receptors in AD.

Vascular amyloid accumulation alters the gabaergic synapse and induces hyperactivity in a model of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is typified by the cerebrovascular deposition of amyloid. The mechanisms underlying the contribution of CAA to neurodegeneration are not currently understood. Although CAA is highly associated with the accumulation of β‐amyloid (Aβ), other amyloids are known to associate with the vasculature. Alzheimer's disease (AD) is characterized by parenchymal Aβ deposition and intracellular accumulation of tau as neurofibrillary tangles (NFTs), affecting synapses directly, leading to behavioral and physical impairment. CAA increases with age and is present in 70%–97% of individuals with AD. Studies have overwhelmingly focused on the connection between parenchymal amyloid accumulation and synaptotoxicity; thus, the contribution of vascular amyloid is mostly understudied. Here, synaptic alterations induced by vascular amyloid accumulation and their behavioral consequences were characterized using a mouse model of Familial Danish dementia (FDD), a neurodegenerative disease characterized by the accumulation of Danish amyloid (ADan) in the vasculature. The mouse model (Tg‐FDD) displays a hyperactive phenotype that potentially arises from impairment in the GABAergic synapses, as determined by electrophysiological analysis. We demonstrated that the disruption of GABAergic synapse organization causes this impairment and provided evidence that GABAergic synapses are impaired in patients with CAA pathology. Understanding the mechanism that CAA contributes to synaptic dysfunction in AD‐related dementias is of critical importance for developing future therapeutic interventions.

Explicating anti-amyloidogenic role of curcumin and piperine via amyloid beta (Aβ) explicit pathway: recovery and reversal paradigm effects

Previously, we reported the synergistic effects of curcumin and piperine in cell cultures as potential anti-cholinesterase and anti-amyloidogenic agents. Due to limited findings on the enrolment of these compounds on epigenetic events in AD, we aimed at elucidating the expression profiles of Aβ42-induced SH-SY5Y cells using microarray profiling. In this study, an optimized concentration of 35 µM of curcumin and piperine in combination was used to treat Aβ42 fibril and high-throughput microarray profiling was performed on the extracted RNA. This was then compared to curcumin and piperine used singularly at 49.11 µM and 25 µM, respectively. Our results demonstrated that in the curcumin treated group, from the top 10 upregulated and top 10 downregulated significantly differentially expressed genes (p < 0.05; fold change ≥ 2 or ≤ -2), there were five upregulated and three downregulated genes involved in the amyloidogenic pathway. While from top 10 upregulated and top 10 downregulated significantly differentially expressed genes (p < 0.05; fold change ≥ 2 or ≤ - 2) in the piperine treated group, there were four upregulated and three downregulated genes involved in the same pathway, whereas there were five upregulated and two downregulated genes involved (p < 0.05; fold change ≥ 2 or ≤ - 2) in the curcumin-piperine combined group. Four genes namely GABARAPL1, CTSB, RAB5 and AK5 were expressed significantly in all groups. Other genes such as ITPR1, GSK3B, PPP3CC, ERN1, APH1A, CYCS and CALM2 were novel putative genes that are involved in the pathogenesis of AD. We revealed that curcumin and piperine have displayed their actions against Aβ via the modulation of various mechanistic pathways. Alterations in expression profiles of genes in the neuronal cell model may explain Aβ pathology post-treatment and provide new insights for remedial approaches of a combined treatment using curcumin and piperine.

Synaptic Organizers in Alzheimer's Disease: A Classification Based on Amyloid-β Sensitivity

Synaptic pathology is one of the major hallmarks observed from the early stage of Alzheimer’s disease (AD), leading to cognitive and memory impairment characteristic of AD patients. Synaptic connectivity and specificity are regulated by multiple trans-bindings between pre- and post-synaptic organizers, the complex of which exerts synaptogenic activity. Neurexins (NRXs) and Leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) are the major presynaptic organizers promoting synaptogenesis through their distinct binding to a wide array of postsynaptic organizers. Recent studies have shown that amyloid-β oligomers (AβOs), a major detrimental molecule in AD, interact with NRXs and neuroligin-1, an NRX-binding postsynaptic organizer, to cause synaptic impairment. On the other hand, LAR-RPTPs and their postsynaptic binding partners have no interaction with AβOs, and their synaptogenic activity is maintained even in the presence of AβOs. Here, we review the current evidence regarding the involvement of synaptic organizers in AD, with a focus on Aβ synaptic pathology, to propose a new classification where NRX-based and LAR-RPTP-based synaptic organizing complexes are classified into Aβ-sensitive and Aβ-insensitive synaptic organizers, respectively. We further discuss how their different Aβ sensitivity is involved in Aβ vulnerability and tolerance of synapses for exploring potential therapeutic approaches for AD.

In Vitro and In Vivo Neuroprotective Effects of Etifoxine in β-Amyloidinduced Toxicity Models

Aim:

The aim of this study is to examine the effect of etifoxine on β-amyloid-induced toxicity models.

Background:

Etifoxine is an anxiolytic compound with a dual mechanism of action; it is a positive allosteric modulator of GABAergic receptors as well as a ligand for the 18 kDa mitochondrial Translocator Protein (TSPO). TSPO has recently raised interest in Alzheimer’s Disease (AD), and experimental studies have shown that some TSPO ligands could induce neuroprotective effects in animal models.

Objective:

In this study, we examined the potential protective effect of etifoxine in an in vitro and an in vivo model of amyloid beta (Aβ)-induced toxicity in its oligomeric form, which is a crucial factor in AD pathologic mechanisms.

Method:

Neuronal cultures were intoxicated with Aβ1-42, and the effects of etifoxine on oxidative stress, Tau-hyperphosphorylation and synaptic loss were quantified. In a mice model, behavioral deficits induced by intracerebroventricular administration of Aβ25-35 were measured in a spatial memory test, the spontaneous alternation and in a contextual memory test, the passive avoidance test.

Results:

In neuronal cultures intoxicated with Aβ1-42, etifoxine dose-dependently decreased oxidative stress (methionine sulfoxide positive neurons), tau-hyperphosphorylation and synaptic loss (ratio PSD95/synaptophysin). In a mice model, memory impairments were fully alleviated by etifoxine administered at anxiolytic doses (12.5-50mg/kg). In addition, markers of oxidative stress and apoptosis were decreased in the hippocampus of these animals.

Conclusion:

Our results have shown that in these two models, etifoxine could fully prevent neurotoxicity and pathological changes induced by Aβ. These results confirm that TSPO ligands could offer an interesting therapeutic approach to Alzheimer’s disease.

Learning Promotes Subfield-Specific Synaptic Diversity in Hippocampal CA1 Neurons

The hippocampus is functionally heterogeneous between the dorsal and ventral subfields with left–right asymmetry. To determine the possible location of contextual memory, we performed an inhibitory avoidance task to analyze synaptic plasticity using slice patch-clamp technique. The training bilaterally increased the AMPA/NMDA ratio at dorsal CA3–CA1 synapses, whereas the training did not affect the ratio at ventral CA3–CA1 synapses regardless of the hemisphere. Moreover, sequential recording of miniature excitatory postsynaptic currents and miniature inhibitory postsynaptic currents from the same CA1 neuron clearly showed learning-induced synaptic plasticity. In dorsal CA1 neurons, the training dramatically strengthened both excitatory and inhibitory postsynaptic responses in both hemispheres, whereas the training did not promote the plasticity in either hemisphere in ventral CA1 neurons. Nonstationary fluctuation analysis further revealed that the training bilaterally increased the number of AMPA or GABA_A receptor channels at dorsal CA1 synapses, but not at ventral CA1 synapses, suggesting functional heterogeneity of learning-induced receptor mobility. Finally, the performance clearly impaired by the bilateral microinjection of plasticity blockers in dorsal, but not ventral CA1 subfields, suggesting a crucial role for contextual learning. The quantification of synaptic diversity in specified CA1 subfields may help us to diagnose and evaluate cognitive disorders at the information level.

Pathophysiological Mechanisms of Cognitive Impairment and Neurodegeneration by Toxoplasma gondii Infection

Toxoplasma gondii is an obligate intracellular parasite considered one of the most successful pathogens in the world, owing to its ability to produce long-lasting infections and to persist in the central nervous system (CNS) in most warm-blooded animals, including humans. This parasite has a preference to invade neurons and affect the functioning of glial cells. This could lead to neurological and behavioral changes associated with cognitive impairment. Although several studies in humans and animal models have reported controversial results about the relationship between toxoplasmosis and the onset of dementia as a causal factor, two recent meta-analyses have shown a relative association with Alzheimer’s disease (AD). AD is characterized by amyloid-β (Aβ) peptide accumulation, neurofibrillary tangles, and neuroinflammation. Different authors have found that toxoplasmosis may affect Aβ production in brain areas linked with memory functioning, and can induce a central immune response and neurotransmitter imbalance, which in turn, affect the nervous system microenvironment. In contrast, other studies have revealed a reduction of Aβ plaques and hyperphosphorylated tau protein formation in animal models, which might cause some protective effects. The aim of this article is to summarize and review the newest data in regard to different pathophysiological mechanisms of cerebral toxoplasmosis and their relationship with the development of AD and cognitive impairment. All these associations should be investigated further through clinical and experimental studies.

A mechanistic hypothesis for the impairment of synaptic plasticity by soluble Aβ oligomers from Alzheimer's brain

It is increasingly accepted that early cognitive impairment in Alzheimer's disease results in considerable part from synaptic dysfunction caused by the accumulation of a range of oligomeric assemblies of amyloid β-protein (Aβ). Most studies have used synthetic Aβ peptides to explore the mechanisms of memory deficits in rodent models, but recent work suggests that Aβ assemblies isolated from human (AD) brain tissue are far more potent and disease-relevant. Although reductionist experiments show Aβ oligomers to impair synaptic plasticity and neuronal viability, the responsible mechanisms are only partly understood. Glutamatergic receptors, GABAergic receptors, nicotinic receptors, insulin receptors, the cellular prion protein, inflammatory mediators, and diverse signaling pathways have all been suggested. Studies using AD brain-derived soluble Aβ oligomers suggest that only certain bioactive forms (principally small, diffusible oligomers) can disrupt synaptic plasticity, including by binding to plasma membranes and changing excitatory-inhibitory balance, perturbing mGluR, PrP, and other neuronal surface proteins, down-regulating glutamate transporters, causing glutamate spillover, and activating extrasynaptic GluN2B-containing NMDA receptors. We synthesize these emerging data into a mechanistic hypothesis for synaptic failure in Alzheimer's disease that can be modified as new knowledge is added and specific therapeutics are developed.

Hippocampal long-term synaptic depression and memory deficits induced in early amyloidopathy are prevented by enhancing G-protein-gated inwardly rectifying potassium channel activity

Hippocampal synaptic plasticity disruption by amyloid‐β (Aβ) peptides + thought to be responsible for learning and memory impairments in Alzheimer's disease (AD) early stage. Failures in neuronal excitability maintenance seems to be an underlying mechanism. G‐protein‐gated inwardly rectifying potassium (GirK) channels control neural excitability by hyperpolarization in response to many G‐protein‐coupled receptors activation. Here, in early in vitro and in vivo amyloidosis mouse models, we study whether GirK channels take part of the hippocampal synaptic plasticity impairments generated by Aβ_1–42. In vitro electrophysiological recordings from slices showed that Aβ_1–42 alters synaptic plasticity by switching high‐frequency stimulation (HFS) induced long‐term potentiation (LTP) to long‐term depression (LTD), which led to in vivo hippocampal‐dependent memory deficits. Remarkably, selective pharmacological activation of GirK channels with ML297 rescued both HFS‐induced LTP and habituation memory from Aβ_1–42 action. Moreover, when GirK channels were specifically blocked by Tertiapin‐Q, their activation with ML297 failed to rescue LTP from the HFS‐dependent LTD induced by Aβ_1–42. On the other hand, the molecular analysis of the recorded slices by western blot showed that the expression of GIRK1/2 subunits, which form the prototypical GirK channel in the hippocampus, was not significantly regulated by Aβ_1–42. However, immunohistochemical examination of our in vivo amyloidosis model showed Aβ_1–42 to down‐regulate hippocampal GIRK1 subunit expression. Together, our results describe an Aβ‐mediated deleterious synaptic mechanism that modifies the induction threshold for hippocampal LTP/LTD and underlies memory alterations observed in amyloidosis models. In this scenario, GirK activation assures memory formation by preventing the transformation of HFS‐induced LTP into LTD.

Temporal dynamics of learning-promoted synaptic diversity in CA1 pyramidal neurons

Although contextual learning requires plasticity at both excitatory and inhibitory (E/I) synapses in cornu ammonis 1 (CA1) neurons, the temporal dynamics across the neuronal population are poorly understood. Using an inhibitory avoidance task, we analyzed the dynamic changes in learning-induced E/I synaptic plasticity. The training strengthened GABA_A receptor–mediated synapses within 1 min, peaked at 10 min, and lasted for over 60 min. The intracellular loop (Ser^408−409) of GABA_A receptor β₃ subunit was also phosphorylated within 1 min of training. As the results of strengthening of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor–mediated synapses, CA1 pyramidal neurons exhibited broad diversity of E/I synaptic currents within 5 min. Moreover, presynaptic glutamate release probability at basal dendrites also increased within 5 min. To further quantify the diversified E/I synaptic currents, we calculated self-entropy (bit) for individual neurons. The neurons showed individual levels of the parameter, which rapidly increased within 1 min of training and maintained for over 60 min. These results suggest that learning-induced synaptic plasticity is critical immediately following encoding rather than during the retrieval phase of the learning. Understanding the temporal dynamics along with the quantification of synaptic diversity would be necessary to identify a failure point for learning-promoted plasticity in cognitive disorders.—Sakimoto, Y., Kida, H., Mitsushima, D. Temporal dynamics of learning-promoted synaptic diversity in CA1 pyramidal neurons.

Linking Molecular Pathways and Large-Scale Computational Modeling to Assess Candidate Disease Mechanisms and Pharmacodynamics in Alzheimer's Disease

Introduction: While the prevalence of neurodegenerative diseases associated with dementia such as Alzheimer's disease (AD) increases, our knowledge on the underlying mechanisms, outcome predictors, or therapeutic targets is limited. In this work, we demonstrate how computational multi-scale brain modeling links phenomena of different scales and therefore identifies potential disease mechanisms leading the way to improved diagnostics and treatment. Methods: The Virtual Brain (TVB; thevirtualbrain.org) neuroinformatics platform allows standardized large-scale structural connectivity-based simulations of whole brain dynamics. We provide proof of concept for a novel approach that quantitatively links the effects of altered molecular pathways onto neuronal population dynamics. As a novelty, we connect chemical compounds measured with positron emission tomography (PET) with neural function in TVB addressing the phenomenon of hyperexcitability in AD related to the protein amyloid beta (Abeta). We construct personalized virtual brains based on an averaged healthy connectome and individual PET derived distributions of Abeta in patients with mild cognitive impairment (MCI, N = 8) and Alzheimer's Disease (AD, N = 10) and in age-matched healthy controls (HC, N = 15) using data from ADNI-3 data base (http://adni.loni.usc.edu). In the personalized virtual brains, individual Abeta burden modulates regional Excitation-Inhibition balance, leading to local hyperexcitation with high Abeta loads. We analyze simulated regional neural activity and electroencephalograms (EEG). Results: Known empirical alterations of EEG in patients with AD compared to HCs were reproduced by simulations. The virtual AD group showed slower frequencies in simulated local field potentials and EEG compared to MCI and HC groups. The heterogeneity of the Abeta load is crucial for the virtual EEG slowing which is absent for control models with homogeneous Abeta distributions. Slowing phenomena primarily affect the network hubs, independent of the spatial distribution of Abeta. Modeling the N-methyl-D-aspartate (NMDA) receptor antagonism of memantine in local population models, reveals potential functional reversibility of the observed large-scale alterations (reflected by EEG slowing) in virtual AD brains. Discussion: We demonstrate how TVB enables the simulation of systems effects caused by pathogenetic molecular candidate mechanisms in human virtual brains.

Association of AEBP1 and NRN1 RNA expression with Alzheimer's disease and neurofibrillary tangle density in middle temporal gyrus

We explored RNA expression changes in the middle temporal gyrus (MTG) of Alzheimer's Disease patients (AD) by RNA sequencing the whole transcriptome of 8 AD and 8 Non-Demented (ND) controls. We used three additional expression datasets from related brain regions to validate the findings. The results highlighted the upregulation of AEBP1 and downregulation of NRN1 in AD, as well as their association with Braak staging and neurofibrillary tangles density. Furthermore, more than 400 protein-coding RNAs enriched for "Clathrin-mediated endocytosis" were validated in independent datasets from the same brain region. Finally, using in silico prediction analysis we found a signature of 52 non-protein coding RNAs that perturb key pathways involved in GABAergic transmission and peptide chain elongation. The association of AEBP1 in our data confirmed other published work examining gene expression in the hippocampus of AD patients. NRN1 is involved in neurite outgrowth, and in previous studies it has been shown to reverse synaptic defects and cognitive function impairment in Tg2576 mice. Finally, our results on non-protein coding RNAs suggest a role of these transcripts in altering synaptic and amyloid-β associated pathways.

Neuroprotective and Anti-Amyloid β Effect and Main Chemical Profiles of White Tea: Comparison Against Green, Oolong and Black Tea

White tea (WT) is one of six tea types originally derived from Fujian Province, China. White tea is known for its health-promoting properties. However, the neuroprotective and anti-aggregatory properties of WT against the hallmark toxic Alzheimer's protein, Aβ have not been investigated. In this study, WT, green tea (GT), oolong tea (OT) and black tea (BT) were manufactured using tea leaves from the cultivar Camellia sinensis (Jin Guanyin). The protective effects of these tea extracts were then studied under oxidative stress conditions via t-bhp and H₂O₂ exposure, in addition to Aβ treatment using a PC-12 cell model. Each tea type failed to rescue PC-12 cells from either t-bhp or H₂O₂-mediated toxicity, however each extract exerted significant protection against Aβ-evoked neurotoxicity. Results of the Thioflavin T Kinetic (ThT) and TEM assay showed that Aβ aggregate formation was inhibited by each tea type. Additionally, TEM also supported the different anti-aggregatory effect of WT by modifying Aβ into an amorphous and punctate aggregate morphology. Higher accumulated precedent or potential neuroprotective compounds in WT, including ECG''3Me, 8-C-ascorbyl-EGCG, GABA and Gln, in addition to flavonol or flavone glycosides detected by using UPLC-QTOF-MS and UPLC-QqQ-MS, may contribute to a favourable anti-aggregative and neuroprotective effect of WT against Aβ.