Inhibitory interneuron deficit links altered network activity and cognitive dysfunction in Alzheimer model

Electroencephalography characteristics related to risk of sudden unexpected death in epilepsy in patients with Dravet syndrome

Objective

To investigate the quantitative electroencephalography (EEG) features associated with a high risk of sudden unexpected death in epilepsy (SUDEP) in patients with Dravet syndrome (DS).

Methods

Patients with DS and healthy controls (HCs) who underwent EEG were included in the study. EEG signals were recorded using a 21 channel digital EEG system, and pre-processed data were analyzed to identify quantitative EEG features associated with a high SUDEP risk. To assess the risk of SUDEP, SUDEP-7 scores were used.

Results

A total of 64 patients with DS [38 males and 26 females, aged: 128.51 ± 75.50 months (range: 23-380 months)], and 13 HCs [7 males and 6 females, aged: 95.46 ± 86.48 months (range: 13-263 months)] were included. For the absolute band power, the theta power was significantly higher in the high-SUDEP group than in the low-SUDEP group in the central brain region. For the relative band power, the theta power was also significantly higher in the high-SUDEP group than in the low-SUDEP group in the central and occipital brain regions. The alpha power was significantly lower in the high-SUDEP group than in the low-SUDEP group in the central and parietal brain regions.

Conclusion

Patients with high SUDEP-7 scores have different EEG features from those with low SUDEP-7 scores, suggesting that EEG may be used as a biomarker of SUDEP in DS.

Significance

Early intervention in patients with DS at a high risk of SUDEP can reduce mortality and morbidity. Patients with high theta band powers warrant high-level supervision.

Structure-function relationship of new peptides activating human Nav1.1

Nav1.1 is an important pharmacological target as this voltage-gated sodium channel is involved in neurological and cardiac syndromes. Channel activators are actively sought to try to compensate for haploinsufficiency in several of these pathologies. Herein we used a natural source of new peptide compounds active on ion channels and screened for drugs capable to inhibit channel inactivation as a way to compensate for decreased channel function. We discovered that JzTx-34 is highly active on Nav1.1 and subsequently performed a full structure-activity relationship investigation to identify its pharmacophore. These experiments will help interpret the mechanism of action of this and formerly identified peptides as well as the future identification of new peptides. We also reveal structural determinants that make natural ICK peptides active against Nav1.1 challenging to synthesize. Altogether, the knowledge gained by this study will help facilitate the discovery and development of new compounds active on this critical ion channel target.

Neurophysiological alterations in mice and humans carrying mutations in APP and PSEN1 genes

BACKGROUND

Studies in animal models of Alzheimer's disease (AD) have provided valuable insights into the molecular and cellular processes underlying neuronal network dysfunction. Whether and how AD-related neurophysiological alterations translate between mice and humans remains however uncertain.

METHODS

We characterized neurophysiological alterations in mice and humans carrying AD mutations in the APP and/or PSEN1 genes, focusing on early pre-symptomatic changes. Longitudinal local field potential recordings were performed in APP/PS1 mice and cross-sectional magnetoencephalography recordings in human APP and/or PSEN1 mutation carriers. All recordings were acquired in the left frontal cortex, parietal cortex, and hippocampus. Spectral power and functional connectivity were analyzed and compared with wildtype control mice and healthy age-matched human subjects.

RESULTS

APP/PS1 mice showed increased absolute power, especially at higher frequencies (beta and gamma) and predominantly between 3 and 6 moa. Relative power showed an overall shift from lower to higher frequencies over almost the entire recording period and across all three brain regions. Human mutation carriers, on the other hand, did not show changes in power except for an increase in relative theta power in the hippocampus. Mouse parietal cortex and hippocampal power spectra showed a characteristic peak at around 8 Hz which was not significantly altered in transgenic mice. Human power spectra showed a characteristic peak at around 9 Hz, the frequency of which was significantly reduced in mutation carriers. Significant alterations in functional connectivity were detected in theta, alpha, beta, and gamma frequency bands, but the exact frequency range and direction of change differed for APP/PS1 mice and human mutation carriers.

CONCLUSIONS

Both mice and humans carrying APP and/or PSEN1 mutations show abnormal neurophysiological activity, but several measures do not translate one-to-one between species. Alterations in absolute and relative power in mice should be interpreted with care and may be due to overexpression of amyloid in combination with the absence of tau pathology and cholinergic degeneration. Future studies should explore whether changes in brain activity in other AD mouse models, for instance, those also including tau pathology, provide better translation to the human AD continuum.

Interictal epileptiform discharges affect memory in an Alzheimer's disease mouse model

Interictal epileptiform discharges (IEDs) are transient abnormal electrophysiological events commonly observed in epilepsy patients but are also present in other neurological diseases, such as Alzheimer's disease (AD). Understanding the role IEDs have on the hippocampal circuit is important for our understanding of the cognitive deficits seen in epilepsy and AD. We characterize and compare the IEDs of human epilepsy patients from microwire hippocampal recording with those of AD transgenic mice with implanted multilayer hippocampal silicon probes. Both the local field potential features and firing patterns of pyramidal cells and interneurons were similar in the mouse and human. We found that as IEDs emerged from the CA3-1 circuits, they recruited pyramidal cells and silenced interneurons, followed by post-IED suppression. IEDs suppressed the incidence and altered the properties of physiological sharp-wave ripples, altered their physiological properties, and interfered with the replay of place field sequences in a maze. In addition, IEDs in AD mice inversely correlated with daily memory performance. Together, our work implies that IEDs may present a common and epilepsy-independent phenomenon in neurodegenerative diseases that perturbs hippocampal-cortical communication and interferes with memory.

Early classification of Alzheimer's disease phenotype based on hippocampal electrophysiology in the TgF344-AD rat model

The hippocampus plays a vital role in navigation, learning, and memory, and is affected in Alzheimer's disease (AD). This study investigated the classification of AD-transgenic rats versus wild-type littermates using electrophysiological activity recorded from the hippocampus at an early, presymptomatic stage of the disease (6 months old) in the TgF344-AD rat model. The recorded signals were filtered into low frequency (LFP) and high frequency (spiking activity) signals, and machine learning classifiers were employed to identify the rat genotype (TG vs. WT). By analyzing specific frequency bands in the low frequency signals and calculating distance metrics between spike trains in the high frequency signals, accurate classification was achieved. Gamma band power emerged as a valuable signal for classification, and combining information from both low and high frequency signals improved the accuracy further. These findings provide valuable insights into the early stage effects of AD on different regions of the hippocampus.

Early molecular layer interneuron hyperactivity triggers Purkinje neuron degeneration in SCA1

Toxic proteinaceous deposits and alterations in excitability and activity levels characterize vulnerable neuronal populations in neurodegenerative diseases. Using in vivo two-photon imaging in behaving spinocerebellar ataxia type 1 (Sca1) mice, wherein Purkinje neurons (PNs) degenerate, we identify an inhibitory circuit element (molecular layer interneurons [MLINs]) that becomes prematurely hyperexcitable, compromising sensorimotor signals in the cerebellum at early stages. Mutant MLINs express abnormally elevated parvalbumin, harbor high excitatory-to-inhibitory synaptic density, and display more numerous synaptic connections on PNs, indicating an excitation/inhibition imbalance. Chemogenetic inhibition of hyperexcitable MLINs normalizes parvalbumin expression and restores calcium signaling in Sca1 PNs. Chronic inhibition of mutant MLINs delayed PN degeneration, reduced pathology, and ameliorated motor deficits in Sca1 mice. Conserved proteomic signature of Sca1 MLINs, shared with human SCA1 interneurons, involved the higher expression of FRRS1L, implicated in AMPA receptor trafficking. We thus propose that circuit-level deficits upstream of PNs are one of the main disease triggers in SCA1.

Alterations of resting-state Gamma frequency characteristics in aging and Alzheimer's disease

Alzheimer's disease (AD) is an important brain disease associated with aging. It involves various functional and structural changes which alter the EEG characteristics. Although numerous studies have found changes in delta, theta, alpha, and beta power, fewer studies have looked at the changes in the resting state EEG gamma activity characteristics in AD. This study aimed to investigate the alterations in the frequency and power values of AD patients' resting-state EEG gamma oscillations compared with healthy elderly and young subjects. We performed Fast Fourier Transform (FFT) on the resting state EEG data from 179 participants, including 59 early stage AD patients, 60 healthy elderly, and 60 healthy young subjects. We averaged FFT performed epochs to investigate the power values in the gamma frequency range (28-48 Hz). We then sorted the peaks of power values in the gamma frequency range, and the average of the identified highest three values was named as the gamma dominant peak frequency. The gamma dominant peak frequency of AD patients (Meyes-opened = 33.4 Hz, Meyes-closed = 32.7 Hz) was lower than healthy elderly (Meyes-opened = 35.5 Hz, Meyes-closed = 35.0 Hz) and healthy young subjects (Meyes-opened = 37.2 Hz, Meyes-closed = 37.0 Hz). These results could be related to AD progression and therefore critical for the recent findings regarding the 40 Hz gamma entrainment because it seems they entrain the gamma frequency of AD towards that of healthy young.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-022-09873-4.

Voltage-Gated Na+ Channels in Alzheimer's Disease: Physiological Roles and Therapeutic Potential

Alzheimer's disease (AD) is the most common cause of dementia and is classically characterized by two major histopathological abnormalities: extracellular plaques composed of amyloid beta (Aβ) and intracellular hyperphosphorylated tau. Due to the progressive nature of the disease, it is of the utmost importance to develop disease-modifying therapeutics that tackle AD pathology in its early stages. Attenuation of hippocampal hyperactivity, one of the earliest neuronal abnormalities observed in AD brains, has emerged as a promising strategy to ameliorate cognitive deficits and abate the spread of neurotoxic species. This aberrant hyperactivity has been attributed in part to the dysfunction of voltage-gated Na+ (Nav) channels, which are central mediators of neuronal excitability. Therefore, targeting Nav channels is a promising strategy for developing disease-modifying therapeutics that can correct aberrant neuronal phenotypes in early-stage AD. This review will explore the role of Nav channels in neuronal function, their connections to AD pathology, and their potential as therapeutic targets.

NLRP3-GABA signaling pathway contributes to the pathogenesis of impulsive-like behaviors and cognitive deficits in aged mice

BACKGROUND

Perioperative neurocognitive disorders (PND), such as delirium and cognitive impairment, are commonly encountered complications in aged patients. The inhibitory neurotransmitter γ-aminobutyric acid (GABA) is aberrantly synthesized from reactive astrocytes following inflammatory stimulation and is implicated in the pathophysiology of neurodegenerative diseases. Additionally, the activation of NOD-like receptor protein 3 (NLRP3) inflammasome is involved in PND. Herein, we aimed to investigate whether the NLRP3-GABA signaling pathway contributes to the pathogenesis of aging mice's PND.

METHODS

24-month-old C57BL/6 and astrocyte-specific NLRP3 knockout male mice were used to establish a PND model via tibial fracture surgery. The monoamine oxidase-B (MAOB) inhibitor selegiline (1 mg/kg) was intraperitoneally administered once a day for 7 days after the surgery. PND, including impulsive-like behaviors and cognitive impairment, was evaluated by open field test, elevated plus maze, and fear conditioning. Thereafter, pathological changes of neurodegeneration were explored by western blot and immunofluorescence assays.

RESULTS

Selegiline administration significantly ameliorated TF-induced impulsive-like behaviors and reduced excessive GABA production in reactive hippocampal astrocytes. Moreover, astrocyte-specific NLRP3 knockout mice reversed TF-induced impulsive-like and cognitive impairment behaviors, decreased GABA levels in reactive astrocytes, ameliorated NLRP3-associated inflammatory responses during the early stage, and restored neuronal degeneration in the hippocampus.

CONCLUSIONS

Our findings suggest that anesthesia and surgical procedures trigger neuroinflammation and cognitive deficits, which may be due to NLRP3-GABA activation in the hippocampus of aged mice.

Deletion of Transferrin Receptor 1 in Parvalbumin Interneurons Induces a Hereditary Spastic Paraplegia-Like Phenotype

Hereditary spastic paraplegia (HSP) is a severe neurodegenerative movement disorder, the underlying pathophysiology of which remains poorly understood. Mounting evidence has suggested that iron homeostasis dysregulation can lead to motor function impairment. However, whether deficits in iron homeostasis are involved in the pathophysiology of HSP remains unknown. To address this knowledge gap, we focused on parvalbumin-positive (PV+) interneurons, a large category of inhibitory neurons in the central nervous system, which play a critical role in motor regulation. The PV+ interneuron-specific deletion of the gene encoding transferrin receptor 1 (TFR1), a key component of the neuronal iron uptake machinery, induced severe progressive motor deficits in both male and female mice. In addition, we observed skeletal muscle atrophy, axon degeneration in the spinal cord dorsal column, and alterations in the expression of HSP-related proteins in male mice with Tfr1 deletion in the PV+ interneurons. These phenotypes were highly consistent with the core clinical features of HSP cases. Furthermore, the effects on motor function induced by Tfr1 ablation in PV+ interneurons were mostly concentrated in the dorsal spinal cord; however, iron repletion partly rescued the motor defects and axon loss seen in both sexes of conditional Tfr1 mutant mice. Our study describes a new mouse model for mechanistic and therapeutic studies relating to HSP and provides novel insights into iron metabolism in spinal cord PV+ interneurons and its role in the regulation of motor functions.SIGNIFICANCE STATEMENT Iron is crucial for neuronal functioning. Mounting evidence suggests that iron homeostasis dysregulation can induce motor function deficits. Transferrin receptor 1 (TFR1) is thought to be the key component in neuronal iron uptake. We found that deletion of Tfr1 in parvalbumin-positive (PV+) interneurons in mice induced severe progressive motor deficits, skeletal muscle atrophy, axon degeneration in the spinal cord dorsal column, and alterations in the expression of hereditary spastic paraplegia (HSP)-related proteins. These phenotypes were highly consistent with the core clinical features of HSP cases and partly rescued by iron repletion. This study describes a new mouse model for the study of HSP and provides novel insights into iron metabolism in spinal cord PV+ interneurons.

Combining aperiodic 1/f slopes and brain simulation: An EEG/MEG proxy marker of excitation/inhibition imbalance in Alzheimer's disease

INTRODUCTION

Accumulation and interaction of amyloid-beta (Aβ) and tau proteins during progression of Alzheimer's disease (AD) are shown to tilt neuronal circuits away from balanced excitation/inhibition (E/I). Current available techniques for noninvasive interrogation of E/I in the intact human brain, for example, magnetic resonance spectroscopy (MRS), are highly restrictive (i.e., limited spatial extent), have low temporal and spatial resolution and suffer from the limited ability to distinguish accurately between different neurotransmitters complicating its interpretation. As such, these methods alone offer an incomplete explanation of E/I. Recently, the aperiodic component of neural power spectrum, often referred to in the literature as the '1/f slope', has been described as a promising and scalable biomarker that can track disruptions in E/I potentially underlying a spectrum of clinical conditions, such as autism, schizophrenia, or epilepsy, as well as developmental E/I changes as seen in aging.

METHODS

Using 1/f slopes from resting-state spectral data and computational modeling, we developed a new method for inferring E/I alterations in AD.

RESULTS

We tested our method on recent freely and publicly available electroencephalography (EEG) and magnetoencephalography (MEG) datasets of patients with AD or prodromal disease and demonstrated the method's potential for uncovering regional patterns of abnormal excitatory and inhibitory parameters.

DISCUSSION

Our results provide a general framework for investigating circuit-level disorders in AD and developing therapeutic interventions that aim to restore the balance between excitation and inhibition.

Cardiometabolic health, menopausal estrogen therapy and the brain: How effects of estrogens diverge in healthy and unhealthy preclinical models of aging

Research in preclinical models indicates that estrogens are neuroprotective and positively impact cognitive aging. However, clinical data are equivocal as to the benefits of menopausal estrogen therapy to the brain and cognition. Pre-existing cardiometabolic disease may modulate mechanisms by which estrogens act, potentially reducing or reversing protections they provide against cognitive decline. In the current review we propose mechanisms by which cardiometabolic disease may alter estrogen effects, including both alterations in actions directly on brain memory systems and actions on cardiometabolic systems, which in turn impact brain memory systems. Consideration of mechanisms by which estrogen administration can exert differential effects dependent upon health phenotype is consistent with the move towards precision or personalized medicine, which aims to determine which treatment interventions will work for which individuals. Understanding effects of estrogens in both healthy and unhealthy models of aging is critical to optimizing the translational link between preclinical and clinical research.

Prenatal dexamethasone exposure induces anxiety- and depressive-like behavior of male offspring rats through intrauterine programming of the activation of NRG1-ErbB4 signaling in hippocampal PV interneurons

Dexamethasone is a commonly used synthetic glucocorticoid in the clinic. As a compound that can cross the placental barrier to promote fetal lung maturation, dexamethasone is extensively used in pregnant women at risk of premature delivery. However, the use of glucocorticoids during pregnancy increases the risk of neurodevelopmental disorders. In the present study, we observed anxiety- and depressive-like behavior changes and hyperexcitability of hippocampal neurons in adult rat offspring with previous prenatal dexamethasone exposure (PDE); the observed changes were related to in utero damage of parvalbumin interneurons. A programmed change in neuregulin 1 (NRG1)-Erb-b2 receptor tyrosine kinase 4 (ErbB4) signaling was the key to the damage of parvalbumin interneurons in the hippocampus of PDE offspring. Anxiety- and depressive-like behavior, NRG1-ErbB4 signaling activation, and damage of parvalbumin interneurons in PDE offspring were aggravated after chronic stress. The intervention of NRG1-ErbB4 signaling contributed to the improvement in dexamethasone-mediated injury to parvalbumin interneurons. These results suggested that PDE might cause anxiety- and depressive-like behavior changes in male rat offspring through the programmed activation of NRG1-ErbB4 signaling, resulting in damage to parvalbumin interneurons and hyperactivity of the hippocampus. Intrauterine programming of neuregulin 1 (NRG1)-Erb-b2 receptor tyrosine kinase 4 (ERBB4) overactivation by dexamethasone mediates anxiety- and depressive-like behavior in male rat offspring.

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.

Microglial extracellular vesicles induce Alzheimer's disease-related cortico-hippocampal network dysfunction

β-Amyloid is one of the main pathological hallmarks of Alzheimer's disease and plays a major role in synaptic dysfunction. It has been demonstrated that β-amyloid can elicit aberrant excitatory activity in cortical-hippocampal networks, which is associated with behavioural abnormalities. However, the mechanism of the spreading of β-amyloid action within a specific circuitry has not been elucidated yet. We have previously demonstrated that the motion of microglia-derived large extracellular vesicles carrying β-amyloid, at the neuronal surface, is crucial for the initiation and propagation of synaptic dysfunction along the entorhinal-hippocampal circuit. Here, using chronic EEG recordings, we show that a single injection of extracellular vesicles carrying β-amyloid into the mouse entorhinal cortex could trigger alterations in the cortical and hippocampal activity that are reminiscent of those found in Alzheimer's disease mouse models and human patients. The development of EEG abnormalities was associated with progressive memory impairment as assessed by an associative (object-place context recognition) and non-associative (object recognition) task. Importantly, when the motility of extracellular vesicles, carrying β-amyloid, was inhibited, the effect on network stability and memory function was significantly reduced. Our model proposes a new biological mechanism based on the extracellular vesicles-mediated progression of β-amyloid pathology and offers the opportunity to test pharmacological treatments targeting the early stages of Alzheimer's disease.

Progressive excitability changes in the medial entorhinal cortex in the 3xTg mouse model of Alzheimer's disease pathology

Alzheimer's disease (AD) is a chronic neurodegenerative disorder that is characterized by memory loss and progressive cognitive impairments. In mouse models of AD pathology, studies have found neuronal and synaptic deficits in the hippocampus, but less is known about what happens in the medial entorhinal cortex (MEC), which is the primary spatial input to the hippocampus and an early site of AD pathology. Here, we measured the neuronal intrinsic excitability and synaptic activity in MEC layer II (MECII) stellate cells, MECII pyramidal cells, and MEC layer III (MECIII) excitatory neurons at early (3 months) and late (10 months) time points in the 3xTg mouse model of AD pathology. At 3 months of age, prior to the onset of memory impairments, we found early hyperexcitability in MECII stellate and pyramidal cells' intrinsic properties, but this was balanced by a relative reduction in synaptic excitation (E) compared to inhibition (I), suggesting intact homeostatic mechanisms regulating activity in MECII. Conversely, MECIII neurons had reduced intrinsic excitability at this early time point with no change in the synaptic E/I ratio. By 10 months of age, after the onset of memory deficits, neuronal excitability of MECII pyramidal cells and MECIII excitatory neurons was largely normalized in 3xTg mice. However, MECII stellate cells remained hyperexcitable and this was further exacerbated by an increased synaptic E/I ratio. This observed combination of increased intrinsically and synaptically generated excitability suggests a breakdown in homeostatic mechanisms specifically in MECII stellate cells at this post-symptomatic time point. Together, these data suggest that the breakdown in homeostatic excitability mechanisms in MECII stellate cells may contribute to the emergence of memory deficits in AD.

Progressive sleep disturbance in various transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia. The relationship between AD and sleep dysfunction has received increased attention over the past decade. The use of genetically engineered mouse models with enhanced production of amyloid beta (Aβ) or hyperphosphorylated tau has played a critical role in the understanding of the pathophysiology of AD. However, their revelations regarding the progression of sleep impairment in AD have been highly dependent on the mouse model used and the specific techniques employed to examine sleep. Here, we discuss the sleep disturbances and general pathology of 15 mouse models of AD. Sleep disturbances covered in this review include changes to NREM and REM sleep duration, bout lengths, bout counts and power spectra. Our aim is to describe in detail the severity and chronology of sleep disturbances within individual mouse models of AD, as well as reveal broader trends of sleep deterioration that are shared among most models. This review also explores a variety of potential mechanisms relating Aβ accumulation and tau neurofibrillary tangles to the progressive deterioration of sleep observed in AD. Lastly, this review offers perspective on how study design might impact our current understanding of sleep disturbances in AD and provides strategies for future research.

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.

Grid cell disruption in a mouse model of early Alzheimer's disease reflects reduced integration of self-motion cues

Converging evidence from human and rodent studies suggests that disrupted grid cell coding in the medial entorhinal cortex (MEC) underlies path integration behavioral deficits during early Alzheimer's disease (AD). However, grid cell firing relies on both self-motion cues and environmental features, and it remains unclear whether disrupted grid coding can account for specific path integration deficits reported during early AD. Here, we report in the J20 transgenic amyloid beta (Aβ) mouse model of early AD that grid cells were spatially unstable toward the center of the arena, had qualitatively different spatial components that aligned parallel to the borders of the environment, and exhibited impaired integration of distance traveled via reduced theta phase precession. Our results suggest that disrupted early AD grid coding reflects reduced integration of self-motion cues but not environmental information via geometric boundaries, providing evidence that grid cell impairments underlie path integration deficits during early AD.

Alterations to parvalbumin-expressing interneuron function and associated network oscillations in the hippocampal - medial prefrontal cortex circuit during natural sleep in AppNL-G-F/NL-G-F mice

In the early stages of Alzheimer's disease (AD), the accumulation of the peptide amyloid-β (Aβ) damages synapses and disrupts neuronal activity, leading to the disruption of neuronal oscillations associated with cognition. This is thought to be largely due to impairments in CNS synaptic inhibition, particularly via parvalbumin (PV)-expressing interneurons that are essential for generating several key oscillations. Research in this field has largely been conducted in mouse models that over-express humanised, mutated forms of AD-associated genes that produce exaggerated pathology. This has prompted the development and use of knock-in mouse lines that express these genes at an endogenous level, such as the App^{NL-G-F/NL-G-F} mouse model used in the present study. These mice appear to model the early stages of Aβ-induced network impairments, yet an in-depth characterisation of these impairments in currently lacking. Therefore, using 16 month-old App^{NL-G-F/NL-G-F} mice, we analysed neuronal oscillations found in the hippocampus and medial prefrontal cortex (mPFC) during awake behaviour, rapid eye movement (REM) and non-REM (NREM) sleep to assess the extent of network dysfunction. No alterations to gamma oscillations were found to occur in the hippocampus or mPFC during either awake behaviour, REM or NREM sleep. However, during NREM sleep an increase in the power of mPFC spindles and decrease in the power of hippocampal sharp-wave ripples was identified. The latter was accompanied by an increase in the synchronisation of PV-expressing interneuron activity, as measured using two-photon Ca²⁺ imaging, as well as a decrease in PV-expressing interneuron density. Furthermore, although changes were detected in local network function of mPFC and hippocampus, long-range communication between these regions appeared intact. Altogether, our results suggest that these NREM sleep-specific impairments represent the early stages of circuit breakdown in response to amyloidopathy.

Nanoelectrochemistry reveals how soluble Aβ42 oligomers alter vesicular storage and release of glutamate

Glutamate (Glu) is the major excitatory transmitter in the nervous system. Impairment of its vesicular release by β-amyloid (Aβ) oligomers is thought to participate in pathological processes leading to Alzheimer's disease. However, it remains unclear whether soluble Aβ42 oligomers affect intravesicular amounts of Glu or their release in the brain, or both. Measurements made in this work on single Glu varicosities with an amperometric nanowire Glu biosensor revealed that soluble Aβ42 oligomers first caused a dramatic increase in vesicular Glu storage and stimulation-induced release, accompanied by a high level of parallel spontaneous exocytosis, ultimately resulting in the depletion of intravesicular Glu content and greatly reduced release. Molecular biology tools and mouse models of Aβ amyloidosis have further established that the transient hyperexcitation observed during the primary pathological stage is mediated by an altered behavior of VGLUT1 responsible for transporting Glu into synaptic vesicles. Thereafter, an overexpression of Vps10p-tail-interactor-1a, a protein that maintains spontaneous release of neurotransmitters by selective interaction with t-SNAREs, resulted in a depletion of intravesicular Glu content, triggering advanced-stage neuronal malfunction. These findings are expected to open perspectives for remediating Aβ42-induced neuronal hyperactivity and neuronal degeneration.

The lncRNA Neat1 is associated with astrocyte reactivity and memory deficits in a mouse model of Alzheimer's disease

Dysregulation of long non-coding RNAs (lncRNAs) have been associated with Alzheimer's disease (AD). However, the functional role of lncRNAs in AD remains unclear. Here, we report a crucial role for the lncRNA Neat1 in astrocyte dysfunction and memory deficits associated with AD. Transcriptomics analysis show abnormally high expression levels of NEAT1 in the brains of AD patients relative to aged-matched healthy controls, with the most significantly elevated levels in glial cells. In a human transgenic APP-J20 (J20) mouse model of AD, RNA-fluorescent in situ hybridization characterization of Neat1 expression in hippocampal astrocyte versus non-astrocyte cell populations revealed a significant increase in Neat1 expression in astrocytes of male, but not female, mice. This corresponded with increased seizure susceptibility in J20 male mice. Interestingly, Neat1 deficiency in the dCA1 in J20 male mice did not alter seizure threshold. Mechanistically, Neat1 deficiency in the dorsal area CA1 of the hippocampus (dCA1) J20 male mice significantly improved hippocampus-dependent memory. Neat1 deficiency also remarkably reduced astrocyte reactivity markers suggesting that Neat1 overexpression is associated with astrocyte dysfunction induced by hAPP/Aβ in the J20 mice. Together, these findings indicate that abnormal Neat1 overexpression may contribute to memory deficits in the J20 AD model not through altered neuronal activity, but through astrocyte dysfunction.

Integrated Proteomics to Understand the Role of Neuritin (NRN1) as a Mediator of Cognitive Resilience to Alzheimer's Disease

The molecular mechanisms and pathways enabling certain individuals to remain cognitively normal despite high levels of Alzheimer's disease (AD) pathology remain incompletely understood. These cognitively normal people with AD pathology are described as preclinical or asymptomatic AD (AsymAD) and appear to exhibit cognitive resilience to the clinical manifestations of AD dementia. Here we present a comprehensive network-based approach from cases clinically and pathologically defined as asymptomatic AD to map resilience-associated pathways and extend mechanistic validation. Multiplex tandem mass tag MS (TMT-MS) proteomic data (n = 7787 proteins) was generated on brain tissue from Brodmann area 6 and Brodmann area 37 (n = 109 cases, n = 218 total samples) and evaluated by consensus weighted gene correlation network analysis. Notably, neuritin (NRN1), a neurotrophic factor previously linked to cognitive resilience, was identified as a hub protein in a module associated with synaptic biology. To validate the function of NRN1 with regard to the neurobiology of AD, we conducted microscopy and physiology experiments in a cellular model of AD. NRN1 provided dendritic spine resilience against amyloid-β (Aβ) and blocked Aβ-induced neuronal hyperexcitability in cultured neurons. To better understand the molecular mechanisms of resilience to Aβ provided by NRN1, we assessed how exogenous NRN1 alters the proteome by TMT-MS (n = 8238 proteins) of cultured neurons and integrated the results with the AD brain network. This revealed overlapping synapse-related biology that linked NRN1-induced changes in cultured neurons with human pathways associated with cognitive resilience. Collectively, this highlights the utility of integrating the proteome from the human brain and model systems to advance our understanding of resilience-promoting mechanisms and prioritize therapeutic targets that mediate resilience to AD.

Tau activation of microglial cGAS-IFN reduces MEF2C-mediated cognitive resilience

Pathological hallmarks of Alzheimer's disease (AD) precede clinical symptoms by years, indicating a period of cognitive resilience before the onset of dementia. Here, we report that activation of cyclic GMP-AMP synthase (cGAS) diminishes cognitive resilience by decreasing the neuronal transcriptional network of myocyte enhancer factor 2c (MEF2C) through type I interferon (IFN-I) signaling. Pathogenic tau activates cGAS and IFN-I responses in microglia, in part mediated by cytosolic leakage of mitochondrial DNA. Genetic ablation of Cgas in mice with tauopathy diminished the microglial IFN-I response, preserved synapse integrity and plasticity and protected against cognitive impairment without affecting the pathogenic tau load. cGAS ablation increased, while activation of IFN-I decreased, the neuronal MEF2C expression network linked to cognitive resilience in AD. Pharmacological inhibition of cGAS in mice with tauopathy enhanced the neuronal MEF2C transcriptional network and restored synaptic integrity, plasticity and memory, supporting the therapeutic potential of targeting the cGAS-IFN-MEF2C axis to improve resilience against AD-related pathological insults.

Levetiracetam Increases Hippocampal Blood Flow in Alzheimer's Disease as Measured by Arterial Spin Labelling MRI

BACKGROUND

Patients with Alzheimer's disease (AD) have an increased risk of developing epileptiform discharges, which is associated with a more rapid rate of progression. This suggests that suppression of epileptiform activity could have clinical benefit in patients with AD.

OBJECTIVE

In the current study, we tested whether acute, intravenous administration of levetiracetam led to changes in brain perfusion as measured with arterial spin labeling MRI (ASL-MRI) in AD.

METHODS

We conducted a double-blind, within-subject crossover design study in which participants with mild AD (n = 9) received placebo, 2.5 mg/kg, and 7.5 mg/kg of LEV intravenously in a random order in three sessions. Afterwards, the participants underwent ASL-MRI.

RESULTS

Analysis of relative cerebral blood flow (rCBF) between 2.5 mg of levetiracetam and placebo showed significant decreases in a cluster that included the posterior cingulate cortex, the precuneus, the posterior part of the cingulate gyrus, while increased cerebral blood flow was found in both temporal lobes involving the hippocampus.

CONCLUSION

Administration of 2.5 mg/kg of LEV in patients without any history of epilepsy leads to changes in rCBF in areas known to be affected in the early stages of AD. These areas may be the focus of the epileptiform activity. Larger studies are needed to confirm the current findings.

Tau in cerebrospinal fluid induces neuronal hyperexcitability and alters hippocampal theta oscillations

Alzheimer's disease (AD) and other tauopathies are characterized by the aggregation of tau into soluble and insoluble forms (including tangles and neuropil threads). In humans, a fraction of both phosphorylated and non-phosphorylated N-terminal to mid-domain tau species, are secreted into cerebrospinal fluid (CSF). Some of these CSF tau species can be measured as diagnostic and prognostic biomarkers, starting from early stages of disease. While in animal models of AD pathology, soluble tau aggregates have been shown to disrupt neuronal function, it is unclear whether the tau species present in CSF will modulate neural activity. Here, we have developed and applied a novel approach to examine the electrophysiological effects of CSF from patients with a tau-positive biomarker profile. The method involves incubation of acutely-isolated wild-type mouse hippocampal brain slices with small volumes of diluted human CSF, followed by a suite of electrophysiological recording methods to evaluate their effects on neuronal function, from single cells through to the network level. Comparison of the toxicity profiles of the same CSF samples, with and without immuno-depletion for tau, has enabled a pioneering demonstration that CSF-tau potently modulates neuronal function. We demonstrate that CSF-tau mediates an increase in neuronal excitability in single cells. We then observed, at the network level, increased input-output responses and enhanced paired-pulse facilitation as well as an increase in long-term potentiation. Finally, we show that CSF-tau modifies the generation and maintenance of hippocampal theta oscillations, which have important roles in learning and memory and are known to be altered in AD patients. Together, we describe a novel method for screening human CSF-tau to understand functional effects on neuron and network activity, which could have far-reaching benefits in understanding tau pathology, thus allowing for the development of better targeted treatments for tauopathies in the future.

Cognitive improvement via a modulated rhythmic pulsed magnetic field in D-galactose-induced accelerated aging mice

Rhythmic physical stimulations have emerged as effective noninvasive intervention strategies in the treatment of pathological cognitive deficits. Transcranial magnetic stimulation (TMS) can regulate neural firing and improve the learning and memory abilities of rodents or patients with cognitive deterioration. However, the effects of elaborate magnetic stimulation with low intensity during aging or other neurological disordering processes on cognitive decline remain unclear. In this study, we developed an elaborate modulated pulsed magnetic field (PMF) stimulation with a complex pattern in the theta repeated frequency and gamma carrier frequency and then determined the effects of this rhythmic PMF on the cognitive function of accelerated aging mice established by chronic subcutaneous injection of D-galactose (D-gal). The results of the Morris water maze (MWM) test showed that mice treated with modulated PMF displayed shorter swimming distance and latency time in the spatial exploration acquisition trial and exhibited a significant preference in the target presumptive platform area in the probe trial, all of which indicated the enhancement in spatial learning and memory abilities upon PMF stimulation of the accelerated aging mice. The novel object recognition (NOR) test results showed a similar tendency as the MWM results although without statistical significance. Further determination of histological structures demonstrated that the cognitive function-related hippocampal CA3 neurons degenerated upon D-gal injection, which could also be partially rescued by PMF application. In comparison with the high-intensity TMS approach, low-intensity magnetic stimulation could be much safer and allow deeper penetration without adverse effects such as seizure. In summary, modulated PMF, even with low intensity, could effectively improve rodent cognitive functions impaired by D-gal-induced accelerated aging, which might provide a new safe therapeutic strategy for cognitive deficits as well as other neurological disorders.

Lateral mammillary body neurons in mouse brain are disproportionately vulnerable in Alzheimer's disease

The neural circuits governing the induction and progression of neurodegeneration and memory impairment in Alzheimer's disease (AD) are incompletely understood. The mammillary body (MB), a subcortical node of the medial limbic circuit, is one of the first brain regions to exhibit amyloid deposition in the 5xFAD mouse model of AD. Amyloid burden in the MB correlates with pathological diagnosis of AD in human postmortem brain tissue. Whether and how MB neuronal circuitry contributes to neurodegeneration and memory deficits in AD are unknown. Using 5xFAD mice and postmortem MB samples from individuals with varying degrees of AD pathology, we identified two neuronal cell types in the MB harboring distinct electrophysiological properties and long-range projections: lateral neurons and medial neurons. lateral MB neurons harbored aberrant hyperactivity and exhibited early neurodegeneration in 5xFAD mice compared with lateral MB neurons in wild-type littermates. Inducing hyperactivity in lateral MB neurons in wild-type mice impaired performance on memory tasks, whereas attenuating aberrant hyperactivity in lateral MB neurons ameliorated memory deficits in 5xFAD mice. Our findings suggest that neurodegeneration may be a result of genetically distinct, projection-specific cellular dysfunction and that dysregulated lateral MB neurons may be causally linked to memory deficits in AD.

Compensatory remodeling of a septo-hippocampal GABAergic network in the triple transgenic Alzheimer's mouse model

BACKGROUND

Alzheimer's disease (AD) is characterized by a progressive loss of memory that cannot be efficiently managed by currently available AD therapeutics. So far, most treatments for AD that have the potential to improve memory target neural circuits to protect their integrity. However, the vulnerable neural circuits and their dynamic remodeling during AD progression remain largely undefined.

METHODS

Circuit-based approaches, including anterograde and retrograde tracing, slice electrophysiology, and fiber photometry, were used to investigate the dynamic structural and functional remodeling of a GABAergic circuit projected from the medial septum (MS) to the dentate gyrus (DG) in 3xTg-AD mice during AD progression.

RESULTS

We identified a long-distance GABAergic circuit that couples highly connected MS and DG GABAergic neurons during spatial memory encoding. Furthermore, we found hyperactivity of DG interneurons during early AD, which persisted into late AD stages. Interestingly, MS GABAergic projections developed a series of adaptive strategies to combat DG interneuron hyperactivity. During early-stage AD, MS-DG GABAergic projections exhibit increased inhibitory synaptic strength onto DG interneurons to inhibit their activities. During late-stage AD, MS-DG GABAergic projections form higher anatomical connectivity with DG interneurons and exhibit aberrant outgrowth to increase the inhibition onto DG interneurons.

CONCLUSION

We report the structural and functional remodeling of the MS-DG GABAergic circuit during disease progression in 3xTg-AD mice. Dynamic MS-DG GABAergic circuit remodeling represents a compensatory mechanism to combat DG interneuron hyperactivity induced by reduced GABA transmission.

Aberrant Dopamine System Function in the Ferrous Amyloid Buthionine (FAB) Rat Model of Alzheimer's Disease

Antipsychotics increase the risk of death in elderly patients with Alzheimer's disease (AD). Thus, there is an immediate need for novel therapies to treat comorbid psychosis in AD. Psychosis has been attributed to a dysregulation of the dopamine system and is associated with aberrant regulation by the hippocampus. Given that the hippocampus is a key site of pathology in AD, we posit that aberrant regulation of the dopamine system may contribute to comorbid psychosis in AD. A ferrous amyloid buthionine (FAB) rodent model was used to model a sporadic form of AD. FAB rats displayed functional hippocampal alterations, which were accompanied by decreases in spontaneous, low-frequency oscillations and increases in the firing rates of putative pyramidal neurons. Additionally, FAB rats exhibited increases in dopamine neuron population activity and augmented responses to the locomotor-inducing effects of MK-801, as is consistent with rodent models of psychosis-like symptomatology. Further, working memory deficits in the Y-maze, consistent with an AD-like phenotype, were observed in FAB rats. These data suggest that the aberrant hippocampal activity observed in AD may contribute to dopamine-dependent psychosis, and that the FAB model may be useful for the investigation of comorbid psychosis related to AD. Understanding the pathophysiology that leads to comorbid psychosis in AD will ultimately lead to the discovery of novel targets for the treatment of this disease.

The hippocampus associated GABAergic neural network impairment in early-stage of Alzheimer's disease

Alzheimer's disease (AD) is the commonest neurodegenerative disease with slow progression. Pieces of evidence suggest that the GABAergic system is impaired in the early stage of AD, leading to hippocampal neuron over-activity and further leading to memory and cognitive impairment in patients with AD. However, the precise impairment mechanism of the GABAergic system on the pathogenesis of AD is still unclear. The impairment of neural networks associated with the GABAergic system is tightly associated with AD. Therefore, we describe the roles played by hippocampus-related GABAergic circuits and their impairments in AD neuropathology. In addition, we give our understand on the process from GABAergic circuit impairment to cognitive and memory impairment, since recent studies on astrocyte in AD plays an important role behind cognition dysfunction caused by GABAergic circuit impairment, which helps better understand the GABAergic system and could open up innovative AD therapy.

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.

Impairments of glutamatergic synaptic transmission in Alzheimer's disease

One of the hallmarks of Alzheimer's disease (AD) is structural cell damage and neuronal death in the brains of affected individuals. As these changes are irreversible, it is important to understand their origins and precursors in order to develop treatment strategies against AD. Here, we review evidence for AD-specific impairments of glutamatergic synaptic transmission by relating evidence from human AD subjects to functional studies in animal models of AD. The emerging picture is that early in the disease, the accumulation of toxic β-amyloid aggregates, particularly dimers and low molecular weight oligomers, disrupts glutamate reuptake, which leads to its extracellular accumulation causing neuronal depolarization. This drives the hyperactivation of neurons and might facilitate neuronal damage and degeneration through glutamate neurotoxicity.

Toward the Identification of Neurophysiological Biomarkers for Alzheimer's Disease in Down Syndrome: A Potential Role for Cross-Frequency Phase-Amplitude Coupling Analysis

Cross-frequency coupling (CFC) mechanisms play a central role in brain activity. Pathophysiological mechanisms leading to many brain disorders, such as Alzheimer's disease (AD), may produce unique patterns of brain activity detectable by electroencephalography (EEG). Identifying biomarkers for AD diagnosis is also an ambition among research teams working in Down syndrome (DS), given the increased susceptibility of people with DS to develop early-onset AD (DS-AD). Here, we review accumulating evidence that altered theta-gamma phase-amplitude coupling (PAC) may be one of the earliest EEG signatures of AD, and therefore may serve as an adjuvant tool for detecting cognitive decline in DS-AD. We suggest that this field of research could potentially provide clues to the biophysical mechanisms underlying cognitive dysfunction in DS-AD and generate opportunities for identifying EEG-based biomarkers with diagnostic and prognostic utility in DS-AD.

Multisensory Flicker Modulates Widespread Brain Networks and Reduces Interictal Epileptiform Discharges in Humans

Modulating brain oscillations has strong therapeutic potential. However, commonly used non-invasive interventions such as transcranial magnetic or direct current stimulation have limited effects on deeper cortical structures like the medial temporal lobe. Repetitive audio-visual stimulation, or sensory flicker, modulates such structures in mice but little is known about its effects in humans. Using high spatiotemporal resolution, we mapped and quantified the neurophysiological effects of sensory flicker in human subjects undergoing presurgical intracranial seizure monitoring. We found that flicker modulates both local field potential and single neurons in higher cognitive regions, including the medial temporal lobe and prefrontal cortex, and that local field potential modulation is likely mediated via resonance of involved circuits. We then assessed how flicker affects pathological neural activity, specifically interictal epileptiform discharges, a biomarker of epilepsy also implicated in Alzheimer's and other diseases. In our patient population with focal seizure onsets, sensory flicker decreased the rate interictal epileptiform discharges. Our findings support the use of sensory flicker to modulate deeper cortical structures and mitigate pathological activity in humans.

Alzheimer's disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression

The synapse has consistently been considered a vulnerable and critical target within Alzheimer's disease, and synapse loss is, to date, one of the main biological correlates of cognitive decline within Alzheimer's disease. This occurs prior to neuronal loss with ample evidence that synaptic dysfunction precedes this, in support of the idea that synaptic failure is a crucial stage within disease pathogenesis. The two main pathological hallmarks of Alzheimer's disease, abnormal aggregates of amyloid or tau proteins, have had demonstrable effects on synaptic physiology in animal and cellular models of Alzheimer's disease. There is also growing evidence that these two proteins may have a synergistic effect on neurophysiological dysfunction. Here, we review some of the main findings of synaptic alterations in Alzheimer's disease, and what we know from Alzheimer's disease animal and cellular models. First, we briefly summarize some of the human evidence to suggest that synapses are altered, including how this relates to network activity. Subsequently, animal and cellular models of Alzheimer's disease are considered, highlighting mouse models of amyloid and tau pathology and the role these proteins may play in synaptic dysfunction, either in isolation or examining how the two pathologies may interact in dysfunction. This specifically focuses on neurophysiological function and dysfunction observed within these animal models, typically measured using electrophysiology or calcium imaging. Following synaptic dysfunction and loss, it would be impossible to imagine that this would not alter oscillatory activity within the brain. Therefore, this review also discusses how this may underpin some of the aberrant oscillatory patterns seen in animal models of Alzheimer's disease and human patients. Finally, an overview of some key directions and considerations in the field of synaptic dysfunction in Alzheimer's disease is covered. This includes current therapeutics that are targeted specifically at synaptic dysfunction, but also methods that modulate activity to rescue aberrant oscillatory patterns. Other important future avenues of note in this field include the role of non-neuronal cell types such as astrocytes and microglia, and mechanisms of dysfunction independent of amyloid and tau in Alzheimer's disease. The synapse will certainly continue to be an important target within Alzheimer's disease for the foreseeable future.

Functional excitatory to inhibitory synaptic imbalance and loss of cognitive performance in people with Alzheimer's disease neuropathologic change

Individuals at distinct stages of Alzheimer's disease (AD) show abnormal electroencephalographic activity, which has been linked to network hyperexcitability and cognitive decline. However, whether pro-excitatory changes at the synaptic level are observed in brain areas affected early in AD, and if they are emergent in MCI, is not clearly known. Equally important, it is not known whether global synaptic E/I imbalances correlate with the severity of cognitive impairment in the continuum of AD. Measuring the amplitude of ion currents of human excitatory and inhibitory synaptic receptors microtransplanted from the hippocampus and temporal cortex of cognitively normal, mildly cognitively impaired and AD individuals into surrogate cells, we found regional differences in pro-excitatory shifts of the excitatory to inhibitory (E/I) current ratio that correlates positively with toxic proteins and degree of pathology, and impinges negatively on cognitive performance scores. Using these data with electrophysiologically anchored analysis of the synapto-proteome in the same individuals, we identified a group of proteins sustaining synaptic function and those related to synaptic toxicity. We also found an uncoupling between the function and expression of proteins for GABAergic signaling in the temporal cortex underlying larger E/I and worse cognitive performance. Further analysis of transcriptomic and in situ hybridization datasets from an independent cohort across the continuum of AD confirm regional differences in pro-excitatory shifts of the E/I balance that correlate negatively with the most recent calibrated composite scores for memory, executive function, language and visuospatial abilities, as well as overall cognitive performance. These findings indicate that early shifts of E/I balance may contribute to loss of cognitive capabilities in the continuum of AD clinical syndrome.

Precocious emergence of cognitive and synaptic dysfunction in 3xTg-AD mice exposed prenatally to ethanol

Alzheimer's disease (AD) is the most common cause of dementia, affecting approximately 50 million people worldwide. Early life risk factors for AD, including prenatal exposures, remain underexplored. Exposure of the fetus to alcohol (ethanol) is not uncommon during pregnancy, and may result in physical, behavioral, and cognitive changes that are first detected during childhood but result in lifelong challenges. Whether or not prenatal ethanol exposure may contribute to Alzheimer's disease risk is not yet known. Here we exposed a mouse model of Alzheimer's disease (3xTg-AD), bearing three dementia-associated transgenes, presenilin1 (PS1M146V), human amyloid precursor protein (APPSwe), and human tau (TauP301S), to ethanol on gestational days 13.5-16.5 using an established binge-type maternal ethanol exposure paradigm. We sought to investigate whether prenatal ethanol exposure resulted in a precocious onset or increased severity of AD progression, or both. We found that a brief binge-type gestational exposure to ethanol during a period of peak neuronal migration to the developing cortex resulted in an earlier onset of spatial memory deficits and behavioral inflexibility in the progeny, as assessed by performance on the modified Barnes maze task. The observed cognitive changes coincided with alterations to both GABAergic and glutamatergic synaptic transmission in layer V/VI neurons, diminished GABAergic interneurons, and increased β-amyloid accumulation in the medial prefrontal cortex. These findings provide the first preclinical evidence for prenatal ethanol exposure as a potential factor for modifying the onset of AD-like behavioral dysfunction and set the groundwork for more comprehensive investigations into the underpinnings of AD-like cognitive changes in individuals with fetal alcohol spectrum disorders.

Neuronal hyperexcitability in the Tg2576 mouse model of Alzheimer's disease - the influence of sleep and noradrenergic transmission

The link between Alzheimer's disease (AD) and network hypersynchrony - manifesting as epileptic activities - received considerable attention in the past decade. However, several questions remain unanswered as to its mechanistic underpinnings. Therefore, our objectives were (1) to better characterise epileptic events in the Tg2576 mouse model throughout the sleep-wake cycle and disease progression via electrophysiological recordings and (2) to explore the involvement of noradrenergic transmission in this pathological hypersynchrony. Over and above confirming the previously described early presence and predominance of epileptic events during rapid-eye-movement (REM) sleep, we also show that these events do not worsen with age and are highly phase-locked to the section of the theta cycle during REM sleep where hippocampal pyramidal cells reach their highest firing probability. Finally, we reveal an antiepileptic mechanism of noradrenergic transmission via α1-adrenoreceptors that could explain the intriguing distribution of epileptic events over the sleep-wake cycle in this model, with potential therapeutic implications in the treatment of the epileptic events occurring in many AD patients.

Environmental enrichment and social isolation modulate inhibitory transmission and plasticity in hippocampal area CA2

Environmental factors are well-accepted to play a complex and interdependent role with genetic factors in learning and memory. The goal of this study was to examine how environmental conditions altered synaptic plasticity in hippocampal area CA2. To do this, we housed adult mice for 3 weeks in an enriched environment (EE) consisting of a larger cage with running wheel, and regularly changed toys, tunnels and treats. We then performed whole-cell or extracellular field recordings in hippocampal area CA2 and compared the synaptic plasticity from EE-housed mice with slices from littermate controls housed in standard environment (SE). We found that the inhibitory transmission recruited by CA3 input stimulation in CA2 was significantly less plastic in EE conditions as compared to SE following an electrical tetanus. We demonstrate that delta-opioid receptor (DOR) mediated plasticity is reduced in EE conditions by direct application of DOR agonist. We show that in EE conditions the overall levels of GABA transmission is reduced in CA2 cells by analyzing inhibition of ErbB4 receptor, spontaneous inhibitory currents and paired-pulse ratio. Furthermore, we report that the effect of EE of synaptic plasticity can be rapidly reversed by social isolation. These results demonstrate how the neurons in hippocampal area CA2 are sensitive to environment and may lead to promising therapeutic targets.

Early impairments of visually-driven neuronal ensemble dynamics in the rTg4510 tauopathy mouse model

Tau protein pathology is a hallmark of many neurodegenerative diseases, including Alzheimer's Disease or frontotemporal dementia. Synaptic dysfunction and abnormal visual evoked potentials have been reported in murine models of tauopathy, but little is known about the state of the network activity on a single neuronal level prior to brain atrophy. In the present study, oscillatory rhythms and single-cell calcium activity of primary visual cortex pyramidal neuron population were investigated in basal and light evoked states in the rTg4510 tauopathy mouse model prior to neurodegeneration. We found a decrease in their responsivity and overall activity which was insensitive to GABAergic modulation. Despite an enhancement of basal state coactivation of cortical pyramidal neurons, a loss of input-output synchronicity was observed. Dysfunction of cortical pyramidal function was also reflected in a reduction of basal theta oscillations and enhanced susceptibility to a sub-convulsive dose of pentylenetetrazol in rTg4510 mice. Our results unveil impairments in visual cortical pyramidal neuron processing and define aberrant oscillations as biomarker candidates in early stages of neurodegenerative tauopathies.

Lamotrigine rescues neuronal alterations and prevents seizure-induced memory decline in an Alzheimer's disease mouse model

Epilepsy is a comorbidity associated with Alzheimer's disease (AD), often starting many years earlier than memory decline. Investigating this association in the early pre-symptomatic stages of AD can unveil new mechanisms of the pathology as well as guide the use of antiepileptic drugs to prevent or delay hyperexcitability-related pathological effects of AD. We investigated the impact of repeated seizures on hippocampal memory and amyloid-β (Aβ) load in pre-symptomatic Tg2576 mice, a transgenic model of AD. Seizure induction caused memory deficits and an increase in oligomeric Aβ₄₂ and fibrillary species selectively in pre-symptomatic transgenic mice, and not in their wildtype littermates. Electrophysiological patch-clamp recordings in ex vivo CA1 pyramidal neurons and immunoblots were carried out to investigate the neuronal alterations associated with the behavioral outcomes of Tg2576 mice. CA1 pyramidal neurons exhibited increased intrinsic excitability and lower hyperpolarization-activated I_h current. CA1 also displayed lower expression of the hyperpolarization-activated cyclic nucleotide-gated HCN1 subunit, a protein already identified as downregulated in the AD human proteome. The antiepileptic drug lamotrigine restored electrophysiological alterations and prevented both memory deficits and the increase in extracellular Aβ induced by seizures. Thus our study provides evidence of pre-symptomatic hippocampal neuronal alterations leading to hyperexcitability and associated with both higher susceptibility to seizures and to AD-specific seizure-induced memory impairment. Our findings also provide a basis for the use of the antiepileptic drug lamotrigine as a way to counteract acceleration of AD induced by seizures in the early phases of the pathology.

Chronic seizures induce sex-specific cognitive deficits with loss of presenilin 2 function

Patients with early-onset Alzheimer's disease (EOAD) are at elevated risk for seizures, including patients with presenilin 2 (PSEN2) variants. Like people with epilepsy, uncontrolled seizures may worsen cognitive function in AD. While the relationship between seizures and amyloid beta accumulation has been more thoroughly investigated, the role of other drivers of seizure susceptibility in EOAD remain relatively understudied. We therefore sought to define the impact of loss of normal PSEN2 function and chronic seizures on cognitive function in the aged brain. Male and female PSEN2 KO and age- and sex-matched wild-type (WT) mice were sham or corneal kindled beginning at 6-months-old. Kindled and sham-kindled mice were then challenged up to 6 weeks later in a battery of cognitive tests: non-habituated open field (OF), T-maze spontaneous alternation (TM), and Barnes maze (BM), followed by immunohistochemistry for markers of neuroinflammation and neuroplasticity. PSEN2 KO mice required significantly more stimulations to kindle (males: p < 0.02; females: p < 0.02) versus WT. Across a range of behavioral tests, the cognitive performance of kindled female PSEN2 KO mice was most significantly impaired versus age-matched WT females. Male BM performance was generally worsened by seizures (p = 0.038), but loss of PSEN2 function did not itself worsen cognitive performance. Conversely, kindled PSEN2 KO females made the most BM errors (p = 0.007). Chronic seizures also significantly altered expression of hippocampal neuroinflammation and neuroplasticity markers in a sex-specific manner. Chronic seizures may thus significantly worsen hippocampus-dependent cognitive deficits in aged female, but not male, PSEN2 KO mice. Our work suggests that untreated focal seizures may worsen cognitive burden with loss of normal PSEN2 function in a sex-related manner.

Interictal epileptiform discharges affect memory in an Alzheimer's Disease mouse model

Interictal epileptiform discharges (IEDs) are transient abnormal electrophysiological events commonly observed in epilepsy patients but are also present in other neurological disease, such as Alzheimer's Disease (AD). Understanding the role IEDs have on the hippocampal circuit is important for our understanding of the cognitive deficits seen in epilepsy and AD. We characterize and compare the IEDs of human epilepsy patients from microwire hippocampal recording with those of AD transgenic mice with implanted multi-layer hippocampal silicon probes. Both the local field potential features and firing patterns of pyramidal cells and interneurons were similar in mouse and human. We found that as IEDs emerged from the CA3-1 circuits, they recruited pyramidal cells and silenced interneurons, followed by post-IED suppression. IEDs suppressed the incidence and altered the properties of physiological sharp-wave ripples (SPW-Rs), altered their physiological properties, and interfered with the replay of place field sequences in a maze. In addition, IEDs in AD mice inversely correlated with daily memory performance. Together, our work implicates that IEDs may present a common and epilepsy-independent phenomenon in neurodegenerative diseases that perturbs hippocampal-cortical communication and interferes with memory.

Significant Statement

Prevalence of neurodegenerative diseases and the number of people with dementia is increasing steadily. Therefore, novel treatment strategies for learning and memory disorders are urgently necessary. IEDs, apart from being a surrogate for epileptic brain regions, have also been linked to cognitive decline. Here we report that IEDs in human epilepsy patients and AD mouse models have similar local field potential characteristics and associated firing patterns of pyramidal cells and interneurons. Mice with more IEDs displayed fewer hippocampal SPW-Rs, poorer replay of spatial trajectories, and decreased memory performance. IED suppression is an unexplored target to treat cognitive dysfunction in neurodegenerative diseases.

The Clustered Gamma Protocadherin Pcdhγc4 Isoform Regulates Cortical Interneuron Programmed Cell Death in the Mouse Cortex

Cortical function critically depends on inhibitory/excitatory balance. Cortical inhibitory interneurons (cINs) are born in the ventral forebrain and migrate into cortex, where their numbers are adjusted by programmed cell death. Previously, we showed that loss of clustered gamma protocadherins (Pcdhγ), but not of genes in the alpha or beta clusters, increased dramatically cIN BAX-dependent cell death in mice. Here we show that the sole deletion of the Pcdhγc4 isoform, but not of the other 21 isoforms in the Pcdhγ gene cluster, increased cIN cell death in mice during the normal period of programmed cell death. Viral expression of the Pcdhγc4 isoform rescued transplanted cINs lacking Pcdhγ from cell death. We conclude that Pcdhγ, specifically Pcdhγc4, plays a critical role in regulating the survival of cINs during their normal period of cell death. This demonstrates a novel specificity in the role of Pcdhγ isoforms in cortical development.

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-β (Aβ) 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 Aβ42-induced degradation of gamma oscillations. Furthermore, the 5×FAD mice lacking gal3 (5×FAD-Gal3KO) exhibited WT-like gamma network dynamics and decreased Aβ 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.

Aberrant protein S-nitrosylation contributes to hyperexcitability-induced synaptic damage in Alzheimer's disease: Mechanistic insights and potential therapies

Alzheimer's disease (AD) is arguably the most common cause of dementia in the elderly and is marked by progressive synaptic degeneration, which in turn leads to cognitive decline. Studies in patients and in various AD models have shown that one of the early signatures of AD is neuronal hyperactivity. This excessive electrical activity contributes to dysregulated neural network function and synaptic damage. Mechanistically, evidence suggests that hyperexcitability accelerates production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) that contribute to neural network impairment and synapse loss. This review focuses on the pathways and molecular changes that cause hyperexcitability and how RNS-dependent posttranslational modifications, represented predominantly by protein S-nitrosylation, mediate, at least in part, the deleterious effects of hyperexcitability on single neurons and the neural network, resulting in synaptic loss in AD.

Circadian time- and sleep-dependent modulation of cortical parvalbumin-positive inhibitory neurons

Parvalbumin-positive neurons (PVs) are the main class of inhibitory neurons in the mammalian central nervous system. By examining diurnal changes in synaptic and neuronal activity of PVs in the supragranular layer of the mouse primary visual cortex (V1), we found that both PV input and output are modulated in a time- and sleep-dependent manner throughout the 24-h day. We first show that PV-evoked inhibition is stronger by the end of the light cycle (ZT12) relative to the end of the dark cycle (ZT0), which is in line with the lower inhibitory input of PV neurons at ZT12 than at ZT0. Interestingly, PV inhibitory and excitatory synaptic transmission slowly oscillate in opposite directions during the light/dark cycle. Although excitatory synapses are predominantly regulated by experience, inhibitory synapses are regulated by sleep, via acetylcholine activating M1 receptors. Consistent with synaptic regulation of PVs, we further show in vivo that spontaneous PV activity displays daily rhythm mainly determined by visual experience, which negatively correlates with the activity cycle of surrounding pyramidal neurons and the dorsal lateral geniculate nucleus-evoked responses in V1. These findings underscore the physiological significance of PV's daily modulation.

BrainWAVE: A Flexible Method for Noninvasive Stimulation of Brain Rhythms across Species

Rhythmic neural activity, which coordinates brain regions and neurons to achieve multiple brain functions, is impaired in many diseases. Despite the therapeutic potential of driving brain rhythms, methods to noninvasively target deep brain regions are limited. Accordingly, we recently introduced a noninvasive stimulation approach using flickering lights and sounds ("flicker"). Flicker drives rhythmic activity in deep and superficial brain regions. Gamma flicker spurs immune function, clears pathogens, and rescues memory performance in mice with amyloid pathology. Here, we present substantial improvements to this approach that is flexible, user-friendly, and generalizable across multiple experimental settings and species. We present novel open-source methods for flicker stimulation across rodents and humans. We demonstrate rapid, cross-species induction of rhythmic activity without behavioral confounds in multiple settings from electrophysiology to neuroimaging. This flicker approach provides an exceptional opportunity to discover the therapeutic effects of brain rhythms across scales and species.

Ventral hippocampal CA1 modulates pain behaviors in mice with peripheral inflammation

Chronic pain is one of the most significant medical problems throughout the world. Recent evidence has confirmed the hippocampus as an active modulator of pain chronicity, but the underlying mechanisms remain unclear. Using in vivo electrophysiology, we identify a neural ensemble in the ventral hippocampal CA1 (vCA1) that shows inhibitory responses to noxious but not innocuous stimuli. Following peripheral inflammation, this ensemble becomes responsive to innocuous stimuli, representing hypersensitivity. Mimicking the inhibition of vCA1 neurons using chemogenetics induces chronic pain-like behaviors in naive mice, whereas activating vCA1 neurons in mice with peripheral inflammation results in a reduction of pain-related behaviors. Pathway-specific manipulation of vCA1 projections to basolateral amygdala (BLA) and infralimbic cortex (IL) shows that these pathways are differentially involved in pain modulation at different temporal stages of chronic inflammatory pain. These results confirm a crucial role of the vCA1 and its circuits in modulating the development of chronic pain.