Inter-network functional connectivity increases by beta-amyloid and may facilitate the early stage of tau accumulation

Alzheimer's disease (AD) is pathologically marked by tau tangles and beta-amyloid (Aβ) plaques. It has been hypothesized that Aβ facilitates spread of tau outside of the medial temporal lobe (MTL), but exact mechanism of this facilitation remains unclear. We aimed to test the hypothesis that abnormal Aβ induces an increase in inter-network functional connectivity, which in turn induces early-stage tau elevation in limbic network. Our study used 18F-Florbetaben Aβ positron emission tomography (PET), 18F-MK6240 tau-PET, and resting-state functional magnetic resonance imaging (rs-fMRI) from 489 healthy unimpaired older adults, including 46 with longitudinal data. We found significant correlations between tau in limbic network and Aβ in distinct functional networks. We then demonstrated that Aβ+ /Tau- participants exhibited elevated inter-network functional connectivity of the limbic network. Finally, our longitudinal results showed that annual increases in inter-network functional connectivity between limbic network and default mode and control networks were linked to annual tau elevation in limbic network, primarily modulated by Aβ+ individuals. Understanding this early brain alteration in response to pathologies could guide treatments early in disease course.

Nanoarchitecture of CaV2.1 channels and GABAB receptors in the mouse hippocampus: Impact of APP/PS1 pathology

Voltage-gated CaV2.1 (P/Q-type) Ca2+ channels play a crucial role in regulating neurotransmitter release, thus contributing to synaptic plasticity and to processes such as learning and memory. Despite their recognized importance in neural function, there is limited information on their potential involvement in neurodegenerative conditions such as Alzheimer's disease (AD). Here, we aimed to explore the impact of AD pathology on the density and nanoscale compartmentalization of CaV2.1 channels in the hippocampus in association with GABAB receptors. Histoblotting experiments showed that the density of CaV2.1 channel was significantly reduced in the hippocampus of APP/PS1 mice in a laminar-dependent manner. CaV2.1 channel was enriched in the active zone of the axon terminals and was present at a very low density over the surface of dendritic tree of the CA1 pyramidal cells, as shown by quantitative SDS-digested freeze-fracture replica labelling (SDS-FRL). In APP/PS1 mice, the density of CaV2.1 channel in the active zone was significantly reduced in the strata radiatum and lacunosum-moleculare, while it remained unaltered in the stratum oriens. The decline in Cav2.1 channel density was found to be associated with a corresponding impairment in the GABAergic synaptic function, as evidenced by electrophysiological experiments carried out in the hippocampus of APP/PS1 mice. Remarkably, double SDS-FRL showed a co-clustering of CaV2.1 channel and GABAB1 receptor in nanodomains (~40-50 nm) in wild type mice, while in APP/PS1 mice this nanoarchitecture was absent. Together, these findings suggest that the AD pathology-induced reduction in CaV2.1 channel density and CaV2.1-GABAB1 de-clustering may play a role in the synaptic transmission alterations shown in the AD hippocampus. Therefore, uncovering these layer-dependent changes in P/Q calcium currents associated with AD pathology can benefit the development of future strategies for AD management.

HCN2 deficiency correlates with memory deficits and hyperexcitability of dCA1 pyramidal neurons in Alzheimer's disease

BACKGROUND

Abnormal excitability of hippocampal neurons may lead to dysfunction of neural circuits and then causes cognitive impairments in Alzheimer's disease (AD). However, the underlying mechanisms remain to be fully elucidated.

METHODS

Electrophysiology was performed to examine the intrinsic excitability of CA1 neurons and the activity of the hyperpolarization-activated cyclic nucleotide-gated ion channels (HCNs) of CA1 neurons in wild type (WT) and hAPP-J20 mice. The activity of CA1 pyramidal neurons (PNs) was modulated with chemogenetics. The activity of HCNs was regulated with nonselective facilitator (cAMP) or inhibitor (ZD7288) of HCNs. Immunohistochemical staining or western blotting were performed to examine the expression of HCN1 and HCN2 in the hippocampus of WT and hAPP-J20 mice, or AD patients and non-AD controls. AAVs were injected to specifically modulate the expression of HCN2 in dorsal CA1 (dCA1) PNs. Cognitive performance of mice was assessed with behavioral tests.

RESULTS

dCA1 PNs were more excitable in hAPP-J20 mice, but the excitability of PNs in the ventral CA1 (vCA1) or PV neurons was comparable between WT and hAPP-J20 mice. The activity of the HCNs was reduced in dCA1 PNs of hAPP-J20 mice, and pharmacologically increasing the activity of HCNs attenuated the hyperexcitability of dCA1 PNs in hAPP-J20 mice, suggesting that the reduced activity of HCNs is associated with the hyperexcitability of dCA1 PNs in hAPP-J20 mice. The expression of HCN2 but not HCN1 was reduced in the hippocampus of hAPP-J20 mice, and the expression of HCN2 was also reduced in the hippocampus of AD patients, suggesting that dysregulation of HCN2 is associated with the reduced activity of HCNs in AD. Overexpressing HCN2 rescued the activity of HCNs, attenuated the hyperexcitability of dCA1 PNs and improved memory of hAPP-J20 mice, and knocking down HCN2 impaired the function of HCNs, increased the excitability of dCA1 PNs and led to memory deficits in WT mice.

CONCLUSIONS

Our data suggest that dysregulation of HCNs, particularly HCN2, contributes to the abnormal excitability of CA1 PNs in AD mice and probably in AD patients as well, and thus provide new insights into the mechanisms underlying the aberrant activity or excitability of hippocampal neurons in AD.

BACE1-dependent cleavage of GABAA receptor contributes to neural hyperexcitability and disease progression in Alzheimer's disease

Neural hyperexcitability has been clinically associated with amyloid-β (Aβ) pathology and cognitive impairment in Alzheimer's disease (AD). Here, we show that decreased GABAA receptor (GABAAR) currents are linked to hippocampal granule cell hyperexcitability in the AD mouse model APP23. Elevated levels of β-secretase (BACE1), the β-secretase responsible for generating Aβ peptides, lead to aberrant cleavage of GABAAR β1/2/3 subunits in the brains of APP23 mice and AD patients. Moreover, BACE1-dependent cleavage of the β subunits leads to a decrease in GABAAR-mediated inhibitory currents in BACE1 transgenic mice. Finally, we show that the neural hyperexcitability, Aβ load, and spatial memory deficit phenotypes of APP23 mice are significantly reduced upon the granule cell expression of a non-cleavable β3 subunit mutant. Collectively, our study establishes that BACE1-dependent cleavage of GABAAR β subunits promotes the pathological hyperexcitability known to drive neurodegeneration and cognitive impairment in the AD brain, suggesting that prevention of the cleavage could slow disease progression.

A central role for acetylcholine in entorhinal cortex function and dysfunction with age in humans and mice

Structural and functional changes in the entorhinal cortex (EC) are among the earliest signs of cognitive aging. Here, we ask whether a compromised cholinergic system influences early EC impairments and plays a primary role in EC cognition. We evaluated the relationship between loss of integrity of cholinergic inputs to the EC and cognitive deficits in otherwise healthy humans and mice. Using in vivo imaging (PET/MRI) in older humans and high-resolution imaging in wild-type mice and mice with genetic susceptibility to Alzheimer's disease pathology, we establish that loss of cholinergic input to the EC is, in fact, an early feature in cognitive aging. Through mechanistic studies in mice, we find a central role for EC-projecting cholinergic neurons in the expression of EC-related behaviors. Our data demonstrate that alterations to the cholinergic EC are an early, conserved feature of cognitive aging across species and may serve as an early predictor of cognitive status.

Postbiotics as a therapeutic tool in Alzheimer's disease: Insights into molecular pathways and neuroprotective effects

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterized by oxidative stress, neuroinflammation, mitochondrial dysfunction, neurotransmitter imbalance, tau hyperphosphorylation, and amyloid beta (Aβ) accumulation in brain regions. The gut microbiota (GM) has a major impact on brain function due to its bidirectional interaction with the gut through the gut-brain axis. The gut dysbiosis has been associated with neurological disorders, emphasizing the importance of gut homeostasis in maintaining appropriate brain function. The changes in the composition of microbiomes influence neuroinflammation and Aβ accumulation by releasing pro-inflammatory cytokines, decreasing gut and blood-brain barrier (BBB) integrity, and microglial activation in the brain. Postbiotics, are bioactive compounds produced after fermentation, have been shown to provide several health benefits, particularly in terms of neuroinflammation and cognitive alterations associated with AD. Several research studies on animal models and human have successfully proven the effects of postbiotics on enhancing cognition and memory in experimental animals. This article explores the protective effects of postbiotics on cellular mechanisms responsible for AD pathogenesis and studies highlighting the influence of postbiotics as a total combination and specific compounds, including short-chain fatty acids (SCFAs). In addition, postbiotics act as a promising option for future research to deal with AD's progressive nature and improve an individual's life quality using microbiota modulation.

Targeting high-affinity nicotinic receptors protects against the functional consequences of β-amyloid in mouse hippocampus

The accumulation of β-amyloid oligomers is a hallmark of Alzheimer's disease, inducing neural and network dysfunction in the early stages of pathology. The hippocampus is affected early in the pathogenesis of AD, however the impact of soluble β-amyloid on the dentate gyrus (DG) subregion of the hippocampus and its interaction with nicotinic acetylcholine receptors (nAChRs) within this region are not known. Using a localized model of over-expression, we show that β-amyloid induces early-onset neuronal hyperactivity and hippocampal-dependent memory deficits in mice. Further, we find the DG region to be under potent and sub-type specific nicotinic control in both healthy and pathophysiological conditions, with targeted receptor inhibition leading to a mnemonic rescue against localized amyloidosis. We show that while neurogenesis and synaptic functions are not severely affected in our model, reducing β2-containing nAChR function is associated with the promotion of young adult-born neurons within the pathological network, suggesting a possible protective mechanism. Our data thus reveal the DG network level changes which occur in the early-stages of β-amyloid accumulation and highlight the downstream consequences of targeted nicotinic neuromodulation.

Amyloid-associated hyperconnectivity drives tau spread across connected brain regions in Alzheimer's disease

In Alzheimer's disease (AD), amyloid-β (Aβ) triggers the aggregation and spreading of tau pathology, which drives neurodegeneration and cognitive decline. However, the pathophysiological link between Aβ and tau remains unclear, which hinders therapeutic efforts to attenuate Aβ-related tau accumulation. Aβ has been found to trigger neuronal hyperactivity and hyperconnectivity, and preclinical research has shown that tau spreads across connected neurons in an activity-dependent manner. Here, we hypothesized that neuronal hyperactivity and hypersynchronicity, resulting in functional connectivity increases, constitute a crucial mechanism by which Aβ facilitates the spreading of tau pathology. By combining Aβ positron emission tomography (PET), resting-state functional magnetic resonance imaging, and longitudinal tau-PET in 69 cognitively normal amyloid-negative controls and 140 amyloid-positive patients covering the AD spectrum, we confirmed that Aβ induces hyperconnectivity of temporal lobe tau epicenters to posterior brain regions that are vulnerable to tau accumulation in AD. This was replicated in an independent sample of 55 controls and 345 individuals with preclinical AD and low cortical tau-PET uptake, suggesting that the emergence of Aβ-related hyperconnectivity precedes neocortical tau spreading . Last, using longitudinal tau-PET and mediation analysis, we confirmed that these Aβ-related connectivity increases in tau epicenters to typical tau-vulnerable brain regions in AD mediated the effect of Aβ on faster tau accumulation, unveiling increased connectivity as a potential causal link between the two AD hallmark pathologies. Together, these findings suggest that Aβ promotes tau spreading by eliciting neuronal hyperconnectivity and that targeting Aβ-related neuronal hyperconnectivity may attenuate tau spreading in AD.

Amyloid-β deposition predicts oscillatory slowing of magnetoencephalography signals and a reduction of functional connectivity over time in cognitively unimpaired adults

With the ongoing developments in the field of anti-amyloid therapy for Alzheimer's disease, it is crucial to better understand the longitudinal associations between amyloid-β deposition and altered network activity in the living human brain. We included 110 cognitively unimpaired individuals (67.9 ± 5.7 years), who underwent [18F]flutemetamol (amyloid-β)-PET imaging and resting-state magnetoencephalography (MEG) recording at baseline and 4-year follow-up. We tested associations between baseline amyloid-β deposition and MEG measures (oscillatory power and functional connectivity). Next, we examined the relationship between baseline amyloid-β deposition and longitudinal MEG measures, as well as between baseline MEG measures and longitudinal amyloid-β deposition. Finally, we assessed associations between longitudinal changes in both amyloid-β deposition and MEG measures. Analyses were performed using linear mixed models corrected for age, sex and family. At baseline, amyloid-β deposition in orbitofrontal-posterior cingulate regions (i.e. early Alzheimer's disease regions) was associated with higher theta (4-8 Hz) power (β = 0.17, P < 0.01) in- and lower functional connectivity [inverted Joint Permutation Entropy (JPEinv) theta, β = -0.24, P < 0.001] of these regions, lower whole-brain beta (13-30 Hz) power (β = -0.13, P < 0.05) and lower whole-brain functional connectivity (JPEinv theta, β = -0.18, P < 0.001). Whole-brain amyloid-β deposition was associated with higher whole-brain theta power (β = 0.17, P < 0.05), lower whole-brain beta power (β = -0.13, P < 0.05) and lower whole-brain functional connectivity (JPEinv theta, β = -0.21, P < 0.001). Baseline amyloid-β deposition in early Alzheimer's disease regions also predicted future oscillatory slowing, reflected by increased theta power over time in early Alzheimer's disease regions and across the whole brain (β = 0.11, β = 0.08, P < 0.001), as well as decreased whole-brain beta power over time (β = -0.04, P < 0.05). Baseline amyloid-β deposition in early Alzheimer's disease regions also predicted a reduction in functional connectivity between these regions and the rest of the brain over time (JPEinv theta, β = -0.07, P < 0.05). Baseline whole-brain amyloid-β deposition was associated with increased whole-brain theta power over time (β = 0.08, P < 0.01). Baseline MEG measures were not associated with longitudinal amyloid-β deposition. Longitudinal changes in amyloid-β deposition in early Alzheimer's disease regions were associated with longitudinal changes in functional connectivity of early Alzheimer's disease regions (JPEinv theta, β = -0.19, P < 0.05) and the whole brain [corrected amplitude envelope correlations alpha (8-13 Hz), β = -0.22, P < 0.05]. Finally, longitudinal changes in whole-brain amyloid-β deposition were associated with longitudinal changes in whole-brain relative theta power (β = 0.21, P < 0.05). Disruptions of oscillatory power and functional connectivity appear to represent early functional consequences of emerging amyloid-β deposition in cognitively unimpaired individuals. These findings suggest a role for neurophysiology in monitoring disease progression and potential treatment effects in pre-clinical Alzheimer's disease.

Novel synaptic markers predict early tau pathology and cognitive deficit in an asymptomatic population at risk of Alzheimer's disease

Cognitive dysfunction in Alzheimer's disease (AD) correlates closely with pathology in the neuronal microtubule-associated protein tau. Tau pathology may spread via neural synapses. In a population of cognitively unimpaired elderly at elevated risk of AD, we investigated four cerebrospinal (CSF) markers of synaptic dysfunction and degeneration. Three of these (SYT1, SNAP25, and ADAM23) are derived from pre-synaptic structures, while ADAM22 reflects post-synaptic changes. All four markers correlated strongly with tau protein measures. In statistical models, SYT1 accounted for more than half the total variance in both total- and P(181)-tau levels. Observed correlations with CSF levels of Alzheimer amyloid-β (Aβ42) were somewhat weaker. In longitudinal data, baseline levels of ADAM22 and ADAM23 robustly predicted increase over time in both total- and P-tau. CSF SYT1 levels also correlated with PET image uptake of tau and (at a trend level) Aβ in areas of interest for early AD pathology. CSF SYT1 and SNAP25 levels correlated inversely with a global psychometric score and several of its domain subscales. In quantitative trait loci analyses, all four synaptic markers were associated with at least one AD genetic risk locus. Upon "staging" participants by their evidence of amyloid and tau pathology (A/T/N framework), the CSF synaptic markers were unexpectedly reduced in participants with CSF evidence of amyloid but not tau pathology. They were clearly elevated, however, in the CSF of persons with indications of both tau and amyloid pathology. These observations provide evidence for clear pre-synaptic degeneration in cognitively unimpaired persons with biomarker evidence of early AD pathology.

Revealing excitation-inhibition imbalance in Alzheimer's disease using multiscale neural model inversion of resting-state functional MRI

BACKGROUND

Alzheimer's disease (AD) is a serious neurodegenerative disorder without a clear understanding of pathophysiology. Recent experimental data have suggested neuronal excitation-inhibition (E-I) imbalance as an essential element of AD pathology, but E-I imbalance has not been systematically mapped out for either local or large-scale neuronal circuits in AD, precluding precise targeting of E-I imbalance in AD treatment.

METHOD

In this work, we apply a Multiscale Neural Model Inversion (MNMI) framework to the resting-state functional MRI data from the Alzheimer's Disease Neuroimaging Initiative (ADNI) to identify brain regions with disrupted E-I balance in a large network during AD progression.

RESULTS

We observe that both intra-regional and inter-regional E-I balance is progressively disrupted from cognitively normal individuals, to mild cognitive impairment (MCI) and to AD. Also, we find that local inhibitory connections are more significantly impaired than excitatory ones and the strengths of most connections are reduced in MCI and AD, leading to gradual decoupling of neural populations. Moreover, we reveal a core AD network comprised mainly of limbic and cingulate regions. These brain regions exhibit consistent E-I alterations across MCI and AD, and thus may represent important AD biomarkers and therapeutic targets. Lastly, the E-I balance of multiple brain regions in the core AD network is found to be significantly correlated with the cognitive test score.

CONCLUSIONS

Our study constitutes an important attempt to delineate E-I imbalance in large-scale neuronal circuits during AD progression, which may facilitate the development of new treatment paradigms to restore physiological E-I balance in AD.

Aberrant neuronal hyperactivation causes an age-dependent behavioral decline in Caenorhabditis elegans

Age-dependent sensory impairment, memory loss, and cognitive decline are generally attributed to neuron loss, synaptic dysfunction, and decreased neuronal activities over time. Concurrently, increased neuronal activity is reported in humans and other organisms during aging. However, it is unclear whether neuronal hyperactivity is the cause of cognitive impairment or a compensatory mechanism of circuit dysfunction. The roundworm Caenorhabditis elegans exhibits age-dependent declines in an associative learning behavior called thermotaxis, in which its temperature preference on a thermal gradient is contingent on food availability during its cultivation. Cell ablation and calcium imaging demonstrate that the major thermosensory circuit consisting of AFD thermosensory neuron and AIY interneuron is relatively intact in aged animals. On the other hand, ablation of either AWC sensory neurons or AIA interneurons ameliorates the age-dependent thermotaxis decline. Both neurons showed spontaneous and stochastic hyperactivity in aged animals, enhanced by reciprocal communication between AWC and AIA via neurotransmitters and neuropeptides. Our findings suggest that AWC and AIA hyperactivity mediates thermotaxis decline in aged animals. Furthermore, dietary modulation could ameliorate age-dependent thermotaxis decline by suppressing neuronal hyperactivity. We propose that aberrantly enhanced, not diminished, neuronal activities can impair the behavior of aged animals.

Mass spectrometry identifies tau C-terminal phosphorylation cluster during neuronal hyperexcitation

Tau is a microtubule-associated protein implicated in Alzheimer's disease (AD) and other neurodegenerative disorders termed tauopathies. Pathological, aggregated forms of tau form neurofibrillary tangles (NFTs), impairing its ability to stabilize microtubules and promoting neurotoxicity. Indeed, NFTs correlate with neuronal loss and cognitive impairment. Hyperphosphorylation of tau is seen in all tauopathies and mirrors disease progression, suggesting an essential role in pathogenesis. However, hyperphosphorylation remains a generic and ill-defined term, obscuring the functional importance of specific sites in different physiological or pathological settings. Here, we focused on global mapping of tau phosphorylation specifically during conditions of neuronal hyperexcitation. Hyperexcitation is a property of AD and other tauopathies linked to human cognitive deficits and increased risk of developing seizures and epilepsy. Moreover, hyperexcitation promotes extracellular secretion and trans-synaptic propagation of tau. Using unbiased mass spectrometry, we identified a novel phosphorylation signature in the C-terminal domain of tau detectable only during neuronal hyperactivity in primary cultured rat hippocampal neurons. These sites influenced tau localization to dendrites as well as the size of excitatory postsynaptic sites. These results demonstrate novel physiological tau functions at synapses and the utility of comprehensive analysis of tau phosphorylation during specific signaling contexts.

Evaluating the Efficacy of Levetiracetam on Non-Cognitive Symptoms and Pathology in a Tau Mouse Model

Background/Objectives: Alzheimer's disease (AD) is marked by amyloid-β plaques and hyperphosphorylated tau neurofibrillary tangles (NFTs), leading to cognitive decline and debilitating non-cognitive symptoms. This study aimed to evaluate compounds from four different classes in a short-term (7-day) study using transgenic tau mice to assess their ability to reduce non-cognitive symptoms. The best candidate was then evaluated for longer exposure to assess non-cognitive symptoms, cognition, and pathology. Methods: Tg4510 mice, expressing mutated human tau (P301L), were administered with levetiracetam, methylphenidate, diazepam, and quetiapine for 7 days at 6 months old, when pathology and cognitive deficits are established. Drugs were given in the diet, and non-cognitive symptoms were evaluated using metabolic cages. Levetiracetam was chosen for longer exposure (3 months) in 3-month-old Tg4510 mice and non-transgenic controls to assess behavior and pathology. Results: After 3 months of diet, levetiracetam mildly reduced tau pathology in the hippocampus but did not improve cognition in Tg4510 mice. Interestingly, it influenced appetite, body weight, anxiety-like behavior, and contextual fear memory in non-transgenic animals but not in Tg4510 mice. Conclusions: While levetiracetam has shown benefits in amyloid deposition models, it had limited effects on tau pathology and behavior in an animal model of tau deposition, which is crucial for AD context. The differential effects on non-transgenic versus Tg4510 mice warrant further investigation.

The Role of Extracellular Vesicles and Microparticles in Central Nervous System Disorders: Mechanisms, Biomarkers, and Therapeutic Potential

Microscopic, membranous vesicles known as extracellular vesicles (EVs) have been proposed to play a role in the mechanisms underlying central nervous system (CNS) diseases. EVs are secreted by a variety of cells, including myeloid, endothelial, microglial, oligodendroglial, and mesenchymal stem cells (MSCs). Body fluids such as plasma, urine, and cerebrospinal fluid (CSF) contain microparticles (MPs). The detection of MPs in CSF may indicate genetic or environmental susceptibility to conditions such as schizophrenia, schizoaffective disorder, and bipolar disorder. MPs of different origins can exhibit changes in specific biomarkers at various stages of the disease, aiding in the diagnosis and monitoring of neurological conditions. However, understanding the role and clinical applications of MPs is complicated by challenges such as their isolation and dual roles within the CNS. In this review, we discuss the history, characteristics, and roles of MPs in CNS diseases. We also provide practical insights for future research and highlight the challenges that obscure the therapeutic potential of MPs.

Hyperactive neuronal networks facilitate tau spread in an Alzheimer's disease mouse model

Pathological tau spreads throughout the brain along neuronal connections in Alzheimer's disease (AD), but the mechanisms that underlie this process are poorly understood. Given the high incidence and deleterious consequences of epileptiform activity in AD, we hypothesized neuronal hyperactivity and seizures are key factors in tau spread. To examine these interactions, we created a novel mouse model involving the cross of targeted recombination in active populations (TRAP) mice and the 5 times familial AD (5XFAD; 5X-TRAP) model allowing for the permanent fluorescent labelling of neuronal activity. To establish a causal role of seizures in tau spread, we seeded mice with human AD brain-derived tau lysate and induced seizures with pentylenetetrazol (PTZ) kindling. Comprehensive brain mapping of tau pathology and neuronal activity revealed that basal hyperactivity in 5X-TRAP mice was associated with increased tau spread, which was exacerbated by seizure induction through activated networks and correlated with memory deficits. Computational modeling revealed that anterograde tau spread was elevated in 5X-TRAP mice and that regional neuronal activity was predictive of tau spread to that brain region. On a cellular level, we found that in both saline and PTZ-treated 5X-TRAP mice, hyperactive neurons disproportionately contributed to the spread of tau. Further, we found that Synaptogyrin-3, a synaptic vesicle protein that interacts with tau, was increased following PTZ kindling in 5X-TRAP mice, possibly indicative of a synaptic mechanism underlying seizure-exacerbated tau spread. Importantly, postmortem AD brain tissue from patients with a history of seizures showed increased tau pathology in patterns indicative of increased spread and increased Synaptogyrin-3 levels compared to those without seizures. Overall, our study identifies neuronal hyperactivity and seizures as key factors underlying the pathobiological and cognitive progression of AD. Therapies targeting these factors should be tested clinically to slow tau spread and AD progression.

Impaired spatial coding and neuronal hyperactivity in the medial entorhinal cortex of aged App NL-G-F mice

The progressive accumulation of amyloid beta (Aβ) pathology in the brain has been associated with aberrant neuronal network activity and poor cognitive performance in preclinical mouse models of Alzheimer's disease (AD). Presently, our understanding of the mechanisms driving pathology-associated neuronal dysfunction and impaired information processing in the brain remains incomplete. Here, we assessed the impact of advanced Aβ pathology on spatial information processing in the medial entorhinal cortex (MEC) of 18-month App NL-G-F/NL- G-F knock-in (APP KI) mice as they explored contextually novel and familiar open field arenas in a two-day, four-session recording paradigm. We tracked single unit firing activity across all sessions and found that spatial information scores were decreased in MEC neurons from APP KI mice versus those in age-matched C57BL/6J controls. MEC single unit spatial representations were also impacted in APP KI mice. Border cell firing preferences were unstable across sessions and spatial periodicity in putative grid cells was disrupted. In contrast, MEC border cells and grid cells in Control mice were intact and stable across sessions. We then quantified the stability of MEC spatial maps across sessions by utilizing a metric based on the Earth Mover's Distance (EMD). We found evidence for increased instability in spatially-tuned APP KI MEC neurons versus Controls when mice were re-exposed to familiar environments and exposed to a novel environment. Additionally, spatial decoding analysis of MEC single units revealed deficits in position and speed coding in APP KI mice in all session comparisons. Finally, MEC single unit analysis revealed a mild hyperactive phenotype in APP KI mice that appeared to be driven by narrow-spiking units (putative interneurons). These findings tie Aβ-associated dysregulation in neuronal firing to disruptions in spatial information processing that may underlie certain cognitive deficits associated with AD.

Functional network disruption in cognitively unimpaired autosomal dominant Alzheimer's disease: a magnetoencephalography study

Understanding the nature and onset of neurophysiological changes, and the selective vulnerability of central hub regions in the functional network, may aid in managing the growing impact of Alzheimer's disease on society. However, the precise neurophysiological alterations occurring in the pre-clinical stage of human Alzheimer's disease remain controversial. This study aims to provide increased insights on quantitative neurophysiological alterations during a true early stage of Alzheimer's disease. Using high spatial resolution source-reconstructed magnetoencephalography, we investigated regional and whole-brain neurophysiological changes in a unique cohort of 11 cognitively unimpaired individuals with pathogenic mutations in the presenilin-1 or amyloid precursor protein gene and a 1:3 matched control group (n = 33) with a median age of 49 years. We examined several quantitative magnetoencephalography measures that have been shown robust in detecting differences in sporadic Alzheimer's disease patients and are sensitive to excitation-inhibition imbalance. This includes spectral power and functional connectivity in different frequency bands. We also investigated hub vulnerability using the hub disruption index. To understand how magnetoencephalography measures change as the disease progresses through its pre-clinical stage, correlations between magnetoencephalography outcomes and various clinical variables like age were analysed. A comparison of spectral power between mutation carriers and controls revealed oscillatory slowing, characterized by widespread higher theta (4-8 Hz) power, a lower posterior peak frequency and lower occipital alpha 2 (10-13 Hz) power. Functional connectivity analyses presented a lower whole-brain (amplitude-based) functional connectivity in the alpha (8-13 Hz) and beta (13-30 Hz) bands, predominantly located in parieto-temporal hub regions. Furthermore, we found a significant hub disruption index for (phase-based) functional connectivity in the theta band, attributed to both higher functional connectivity in 'non-hub' regions alongside a hub disruption. Neurophysiological changes did not correlate with indicators of pre-clinical disease progression in mutation carriers after multiple comparisons correction. Our findings provide evidence that oscillatory slowing and functional connectivity differences occur before cognitive impairment in individuals with autosomal dominant mutations leading to early onset Alzheimer's disease. The nature and direction of these alterations are comparable to those observed in the clinical stages of Alzheimer's disease, suggest an early excitation-inhibition imbalance, and fit with the activity-dependent functional degeneration hypothesis. These insights may prove useful for early diagnosis and intervention in the future.

Periodontitis associated with brain function impairment in middle-aged and elderly individuals with normal cognition

BACKGROUND

The present study aimed to investigate changes in intranetwork functional connectivity (FC) and internetwork FC in middle-aged and elderly individuals with normal cognition (NC) and varying degrees of periodontitis to determine the effects of periodontitis on brain function.

METHODS

Periodontal findings and resting-state functional magnetic resonance imaging data were acquired from 51 subjects with NC. Independent component analysis and correlation analysis were used for the statistical analysis of the data.

RESULTS

Differences in intranetwork FC were observed among groups in the anterior default-mode network (aDMN), dorsal attention network and dorsal sensorimotor network (dSMN). Compared with the nonperiodontitis (NP) group or the mild-periodontitis group, the analysis of internetwork FC showed increased FC between the auditory network and the ventral attention network (VAN), between the aDMN and the salience network (SN), and between the SN and the VAN and decreased FC between the posterior default-mode network and the right frontoparietal network in the moderate-to-severe periodontitis group. Additionally, internetwork FC between the dSMN and the VAN was also increased in the moderate-to-severe periodontitis group compared to the NP group. The altered intra- and internetwork FC were significantly correlated with the periodontal clinical index.

CONCLUSION

Our results confirmed that periodontitis was associated with both intra- and internetwork FC changes even in NC. The present study indicates that periodontitis might be a potential risk factor for brain damage and provides a theoretical clue and a new treatment target for the early prevention of Alzheimer disease.

PLAIN LANGUAGE SUMMARY

Recent research has proposed that periodontitis is a potential risk factor for Alzheimer disease (AD). However, the relationship between periodontitis and the brain function of middle-aged and elderly individuals with normal cognition (NC) remains unclear. Analyzing the effect of periodontitis on brain function in the NC stage can provide clues to AD development and help achieve early prevention of dementia. The present study aimed to investigate changes in brain functional connectivity (FC) in NC with different severity of periodontitis to determine the effects of periodontitis on brain function. Both changed intranetwork FC and internetwork FC were found in the moderate-to-severe periodontitis group, and periodontitis was associated with brain network function impairment in NC. The present study indicates that periodontitis might be a potential risk factor for brain damage even in NC stage, and provides a theoretical clue and a new treatment target for the early prevention of AD.

Failure in a population: Tauopathy disrupts homeostatic set-points in emergent dynamics despite stability in the constituent neurons

Homeostatic regulation of neuronal activity is essential for robust computation; set-points, such as firing rate, are actively stabilized to compensate for perturbations. The disruption of brain function central to neurodegenerative disease likely arises from impairments of computationally essential set-points. Here, we systematically investigated the effects of tau-mediated neurodegeneration on all known set-points in neuronal activity. We continuously tracked hippocampal neuronal activity across the lifetime of a mouse model of tauopathy. We were unable to detect effects of disease in measures of single-neuron firing activity. By contrast, as tauopathy progressed, there was disruption of network-level neuronal activity, quantified by measuring neuronal pairwise interactions and criticality, a homeostatically controlled, ideal computational regime. Deviations in criticality correlated with symptoms, predicted underlying anatomical pathology, occurred in a sleep-wake-dependent manner, and could be used to reliably classify an animal's genotype. This work illustrates how neurodegeneration may disrupt the computational capacity of neurobiological systems.

Breakdown of TMS evoked EEG signal propagation within the default mode network in Alzheimer's disease

BACKGROUND

The neural activity of the Default Mode Network (DMN) is disrupted in patients with In Alzheimer's disease (AD).

OBJECTIVES

We used a novel multimodal approach to track neural signal propagation within the DMN in AD patients.

METHODS

Twenty mild to moderate AD patients were recruited. We used transcranial magnetic stimulation (TMS) pulses to probe with a millisecond time resolution the propagation of evoked electroencephalography (EEG) signal following the neural activation of the Precuneus (PC), which is a key hub area of the DMN. Moreover, functional and structural magnetic resonance imaging (MRI) data were collected to reconstruct individual features of the DMN.

RESULTS

In AD patients a probe TMS pulse applied over the PC evokes an increased local activity unmasking underlying hyperexcitability. In contrast, the EEG evoked neural signal did not propagate efficiently within the DMN showing a remarkable breakdown of signal propagation. fMRI and structural tractography showed that impaired signal propagation was related to the same connectivity matrices derived from DMN BOLD signal and transferred by specific white matter bundles forming the cingulum. These features were not detectable stimulating other areas (left dorsolateral prefrontal cortex) or for different networks (fronto-parietal network). Finally, connectivity breakdown was associated with cognitive impairment, as measured with the Clinical Dementia Rating Scale sum of boxes (CDR-SB).

CONCLUSIONS

TMS-EEG in AD shows both local hyperexcitability and a lack of signal propagation within the DMN. These neurophysiological features also correlate with structural and cognitive attributes of the patients.

SIGNIFICANCE

Neuronavigated TMS-EEG may be used as a novel neurophysiological biomarker of DMN connectivity in AD patients.

β-amyloid's neurotoxic mechanisms as defined by in vitro microelectrode arrays: a review

Alzheimer's disease is characterized by the aggregation of β-amyloid, a pathological feature believed to drive the neuronal loss and cognitive decline commonly seen in the disease. Given the growing prevalence of this progressive neurodegenerative disease, understanding the exact mechanisms underlying this process has become a top priority. Microelectrode arrays are commonly used for chronic, non-invasive recording of both spontaneous and evoked neuronal activity from diverse in vitro disease models and to evaluate therapeutic or toxic compounds. To date, microelectrode arrays have been used to investigate β-amyloids' toxic effects, β-amyloids role in specific pathological features and to assess pharmacological approaches to treat Alzheimer's disease. The versatility of microelectrode arrays means these studies use a variety of methods and investigate different disease models and brain regions. This review provides an overview of these studies, highlighting their disparities and presenting the status of the current literature. Despite methodological differences, the current literature indicates that β-amyloid has an inhibitory effect on synaptic plasticity and induces network connectivity disruptions. β-amyloid's effect on spontaneous neuronal activity appears more complex. Overall, the literature corroborates the theory that β-amyloid induces neurotoxicity, having a progressive deleterious effect on neuronal signaling and plasticity. These studies also confirm that microelectrode arrays are valuable tools for investigating β-amyloid pathology from a functional perspective, helping to bridge the gap between cellular and network pathology and disease symptoms. The use of microelectrode arrays provides a functional insight into Alzheimer's disease pathology which will aid in the development of novel therapeutic interventions.

Structural and functional remodeling of neural networks in β-amyloid driven hippocampal hyperactivity

Early detection of Alzheimer's disease (AD) is essential for improving the patients outcomes and advancing our understanding of disease, allowing for timely intervention and treatment. However, accurate biomarkers are still lacking. Recent evidence indicates that hippocampal hyperexcitability precedes the diagnosis of AD decades ago, can predict cognitive decline. Thus, could hippocampal hyperactivity be a robust biomarker for early-AD, and what drives hippocampal hyperactivity in early-AD? these critical questions remain to be answered. Increasing clinical and experimental studies suggest that early hippocampal activation is closely associated with longitudinal β-amyloid (Aβ) accumulation, Aβ aggregates, in turn, enhances hippocampal activity. Therefore, in this narrative review, we discuss the role of Aβ-induced altered intrinsic neuronal properties as well as structural and functional remodeling of glutamatergic, GABAergic, cholinergic, noradrenergic, serotonergic circuits in hippocampal hyperactivity. In addition, we analyze the available therapies and trials that can potentially be used clinically to attenuate hippocampal hyperexcitability in AD. Overall, the present review sheds lights on the mechanism behind Aβ-induced hippocampal hyperactivity, and highlights that hippocampal hyperactivity could be a robust biomarker and therapeutic target in prodromal AD.

Neuronal hypofunction and network dysfunction in a mouse model at an early stage of tauopathy

INTRODUCTION

It is unclear how early neuronal deficits occur in tauopathies, if these are associated with changes in neuronal network activity, and if they can be alleviated with therapies.

METHODS

To address this, we performed in vivo two-photon Ca2+ imaging in tauopathy mice at 6 versus 12 months, compared to controls, and treated the younger animals with a tau antibody.

RESULTS

Neuronal function was impaired at 6 months but did not deteriorate further at 12 months, presumably because cortical tau burden was comparable at these ages. At 6 months, neurons were mostly hypoactive, with enhanced neuronal synchrony, and had dysregulated responses to stimulus. Ex vivo, electrophysiology revealed altered synaptic transmission and enhanced excitability of motor cortical neurons, which likely explains the altered network activity. Acute tau antibody treatment reduced pathological tau and gliosis and partially restored neuronal function.

DISCUSSION

Tauopathies are associated with early neuronal deficits that can be attenuated with tau antibody therapy.

HIGHLIGHTS

Neuronal hypofunction in awake and behaving mice in early stages of tauopathy. Altered network activity disrupted local circuitry engagement in tauopathy mice. Enhanced neuronal excitability and altered synaptic transmission in tauopathy mice. Tau antibody acutely reduced soluble phospho-tau and improved neuronal function.

Astrocyte transcriptomic analysis identifies glypican 5 downregulation as a contributor to synaptic dysfunction in Alzheimer's disease models

Synaptic dysfunction is an early feature in Alzheimer's disease (AD) and correlates with cognitive decline. Astrocytes are essential regulators of synapses, impacting synapse formation, maturation, elimination and function. To understand if synapse-supportive functions of astrocytes are altered in AD, we used astrocyte BacTRAP mice to generate a comprehensive dataset of hippocampal astrocyte transcriptional alterations in two mouse models of Alzheimer's pathology (APPswe/PS1dE9 and Tau P301S), characterizing sex and age-dependent changes. We found that astrocytes from both models downregulate genes important for synapse regulation and function such as the synapse-maturation factor Glypican 5. This transcriptional signature is shared with human post-mortem AD patients. Manipulating a key component of this signature by in vivo overexpression of Glypican 5 in astrocytes is sufficient to prevent early synaptic dysfunction and improve spatial learning in APPswe/PS1dE9 mice. These findings open new avenues to target astrocytic factors to mitigate AD synaptic dysfunction.

Cerebral hyperactivation across the Alzheimer's disease pathological cascade

Neuronal dysfunction in specific brain regions or across distributed brain networks is a known feature of Alzheimer's disease. An often reported finding in the early stage of the disease is the presence of increased functional MRI (fMRI) blood oxygenation level-dependent signal under task conditions relative to cognitively normal controls, a phenomenon known as 'hyperactivation'. However, research in the past decades yielded complex, sometimes conflicting results. The magnitude and topology of fMRI hyperactivation patterns have been found to vary across the preclinical and clinical spectrum of Alzheimer's disease, including concomitant 'hypoactivation' in some cases. These incongruences are likely due to a range of factors, including the disease stage at which the cohort is examined, the brain areas or networks studied and the fMRI paradigm utilized to evoke these functional abnormalities. Additionally, a perennial question pertains to the nature of hyperactivation in the context of Alzheimer's disease. Some propose it reflects compensatory mechanisms to sustain cognitive performance, while others suggest it is linked to the pathological disruption of a highly regulated homeostatic cycle that contributes to, or even drives, disease progression. Providing a coherent narrative for these empirical and conceptual discrepancies is paramount to develop disease models, understand the synergy between hyperactivation and the Alzheimer's disease pathological cascade and tailor effective interventions. We first provide a comprehensive overview of functional brain changes spanning the course from normal ageing to the clinical spectrum of Alzheimer's disease. We then highlight evidence supporting a close relationship between fMRI hyperactivation and in vivo markers of Alzheimer's pathology. We primarily focus on task-based fMRI studies in humans, but also consider studies using different functional imaging techniques and animal models. We then discuss the potential mechanisms underlying hyperactivation in the context of Alzheimer's disease and provide a testable framework bridging hyperactivation, ageing, cognition and the Alzheimer's disease pathological cascade. We conclude with a discussion of future challenges and opportunities to advance our understanding of the fundamental disease mechanisms of Alzheimer's disease, and the promising development of therapeutic interventions incorporating or aimed at hyperactivation and large-scale functional systems.

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Cannabidiol ameliorates cognitive decline in 5×FAD mouse model of Alzheimer's disease through potentiating the function of extrasynaptic glycine receptors

Emerging evidence supports the therapeutic potential of cannabinoids in Alzheimer's disease (AD), but the underlying mechanism upon how cannabinoids impact brain cognition and AD pathology remains unclear. Here we show that chronic cannabidiol (CBD) administration significantly mitigates cognitive deficiency and hippocampal β-amyloid (Aβ) pathology in 5×FAD mouse model of AD. CBD achieves its curative effect mainly through potentiating the function of inhibitory extrasynaptic glycine receptor (GlyR) in hippocampal dentate gyrus (DG). Based on the in vitro and in vivo electrophysiological recording and calcium imaging, CBD mediated anti-AD effects via GlyR are mainly accomplished by decreasing neuronal hyperactivity of granule cells in the DG of AD mice. Furthermore, the AAV-mediated ablation of DG GlyRα1, or the GlyRα1S296A mutation that exclusively disrupts CBD binding, significantly intercepts the anti-AD effect of CBD. These findings suggest a GlyR dependent mechanism underlying the therapeutic potential of CBD in the treatment of AD.

Early disruption of entorhinal dopamine in a knock-in model of Alzheimer's disease

The entorhinal cortex (EC) is a critical brain area for memory formation, while also the region exhibiting the earliest histological and functional alterations in Alzheimer's disease (AD). The EC thus has been long hypothesized as one of the originating brain areas of AD pathophysiology, although circuit mechanisms causing its selective vulnerability remain poorly understood. We found that dopamine neurons projecting their axons to the lateral EC (LEC), critical for memory formation in healthy brains, become dysfunctional and cause memory impairments in early AD brains. In amyloid precursor protein knock-in mice with associative memory impairment, LEC dopamine activity and associative memory encoding of LEC layer 2/3 neurons were disrupted in parallel from the early pathological stage. Optogenetic reactivation of LEC dopamine fibers, as well as L- DOPA treatment, rescued associative learning behavior. These results suggest that dysfunction of LEC-projecting dopamine neurons underlies memory impairment in AD from early stages, pointing to a need for clinical investigation of LEC dopamine in AD patients.

Subtype-specific neurons from patient iPSCs display distinct neuropathological features of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by massive neuronal loss in the brain. Both cortical glutamatergic neurons and basal forebrain cholinergic neurons (BFCNs) in the AD brain are selectively vulnerable. The degeneration and dysfunction of these two subtypes of neurons are closely associated with the cognitive decline of AD patients. The determination of cellular and molecular mechanisms involved in AD pathogenesis, especially in the early stage, will largely facilitate the understanding of this disease and the development of proper intervention strategies. However, due to the inaccessibility of living neurons in the brains of patients, it remains unclear how cortical glutamatergic neurons and BFCNs respond to pathological stress in the early stage of AD. In this study, we established in vitro differentiation systems that can efficiently differentiate patient-derived iPSCs into BFCNs. We found that AD-BFCNs secreted less Aβ peptide than cortical glutamatergic neurons did, even though the Aβ42/Aβ40 ratio was comparable to that of cortical glutamatergic neurons. To further mimic the neurotoxic niche in AD brain, we treated iPSC-derived neurons with Aβ42 oligomer (AβO). BFCNs are less sensitive to AβO induced tau phosphorylation and expression than cortical glutamatergic neurons. However, AβO could trigger apoptosis in both AD-cortical glutamatergic neurons and AD-BFCNs. In addition, AD iPSC-derived BFCNs and cortical glutamatergic neurons exhibited distinct electrophysiological firing patterns and elicited different responses to AβO treatment. These observations revealed that subtype-specific neurons display distinct neuropathological changes during the progression of AD, which might help to understand AD pathogenesis at the cellular level.

Alzheimer's disease induced neurons bearing PSEN1 mutations exhibit reduced excitability

Alzheimer's disease (AD) is a devastating neurodegenerative condition that affects memory and cognition, characterized by neuronal loss and currently lacking a cure. Mutations in PSEN1 (Presenilin 1) are among the most common causes of early-onset familial AD (fAD). While changes in neuronal excitability are believed to be early indicators of AD progression, the link between PSEN1 mutations and neuronal excitability remains to be fully elucidated. This study examined iPSC-derived neurons (iNs) from fAD patients with PSEN1 mutations S290C or A246E, alongside CRISPR-corrected isogenic cell lines, to investigate early changes in excitability. Electrophysiological profiling revealed reduced excitability in both PSEN1 mutant iNs compared to their isogenic controls. Neurons bearing S290C and A246E mutations exhibited divergent passive membrane properties compared to isogenic controls, suggesting distinct effects of PSEN1 mutations on neuronal excitability. Additionally, both PSEN1 backgrounds exhibited higher current density of voltage-gated potassium (Kv) channels relative to their isogenic iNs, while displaying comparable voltage-gated sodium (Nav) channel current density. This suggests that the Nav/Kv imbalance contributes to impaired neuronal firing in fAD iNs. Deciphering these early cellular and molecular changes in AD is crucial for understanding disease pathogenesis.

Lateral Entorhinal Cortex Dysfunction in Alzheimer's Disease Mice

In Alzheimer's disease (AD), the formation of amyloid beta and neurofibrillary tangles (NFTs) leads to neuronal loss in entorhinal cortex (EC), a crucial brain region involved in memory and navigation. These pathological changes are concurrent with the onset of memory-related issues in AD patients with symptoms of forgetfulness such as misplacing items, disorientation in familiar environments etc. The lateral EC (LEC) is associated with non-spatial memory processing including object recognition. Since in LEC, neurons fire in response to objects (object cells) and at locations previously occupied by objects (trace cells), pathology in this region could lead to dysfunction in object location coding. In this paper we show that a transgenic mouse model, EC-App/Tau, which expresses both APP and tau primarily in the EC region, have deficits in LEC-specific memory tasks. Using in vivo single-unit electrophysiology recordings we show that the LEC neurons are hyperactive with low information content and high sparsity compared to the controls indicating poor firing fidelity. We finally show that object cells and trace cells fire less precisely in the EC-App/Tau mice compared to controls indicating poor encoding of objects. Overall, we show that AD pathology causes erratic firing of LEC neurons and object coding defects leading to LEC-specific memory impairment.

Co-activation of selective nicotinic acetylcholine receptor subtypes is required to reverse hippocampal network dysfunction and prevent fear memory loss in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia with no known cause and cure. Research suggests that a reduction of GABAergic inhibitory interneurons' activity in the hippocampus by beta-amyloid peptide (Aβ) is a crucial trigger for cognitive impairment in AD via hyperexcitability. Therefore, enhancing hippocampal inhibition is thought to be protective against AD. However, hippocampal inhibitory cells are highly diverse, and these distinct interneuron subtypes differentially regulate hippocampal inhibitory circuits and cognitive processes. Moreover, Aβ unlikely affects all subtypes of inhibitory interneurons in the hippocampus equally. Hence, identifying the affected interneuron subtypes in AD to enhance hippocampal inhibition optimally is conceptually and practically challenging. We have previously found that Aβ selectively binds to two of the three major hippocampal nicotinic acetylcholine receptor (nAChR) subtypes, α7- and α4β2-nAChRs, but not α3β4-nAChRs, and inhibits these two receptors in cultured hippocampal inhibitory interneurons to decrease their activity, leading to hyperexcitation and synaptic dysfunction in excitatory neurons. We have also revealed that co-activation of α7- and α4β2-nAChRs is required to reverse the Aβ-induced adverse effects in hippocampal excitatory neurons. Here, we discover that α7- and α4β2-nAChRs predominantly control the nicotinic cholinergic signaling and neuronal activity in hippocampal parvalbumin-positive (PV+) and somatostatin-positive (SST+) inhibitory interneurons, respectively. Furthermore, we reveal that co-activation of these receptors is necessary to reverse hippocampal network dysfunction and fear memory loss in the amyloid pathology model mice. We thus suggest that co-activation of PV+ and SST+ cells is a novel strategy to reverse hippocampal dysfunction and cognitive decline in AD.

Dynamic entrainment: A deep learning and data-driven process approach for synchronization in the Hodgkin-Huxley model

Resonance and synchronized rhythm are significant phenomena observed in dynamical systems in nature, particularly in biological contexts. These phenomena can either enhance or disrupt system functioning. Numerous examples illustrate the necessity for organs within the human body to maintain their rhythmic patterns for proper operation. For instance, in the brain, synchronized or desynchronized electrical activities can contribute to neurodegenerative conditions like Huntington's disease. In this paper, we utilize the well-established Hodgkin-Huxley (HH) model, which describes the propagation of action potentials in neurons through conductance-based mechanisms. Employing a "data-driven" approach alongside the outputs of the HH model, we introduce an innovative technique termed "dynamic entrainment." This technique leverages deep learning methodologies to dynamically sustain the system within its entrainment regime. Our findings show that the results of the dynamic entrainment technique match with the outputs of the mechanistic (HH) model.

Lateral Piezoelectricity of Alzheimer's Aβ Aggregates

Alzheimer's disease (AD) is the most frequent neurodegenerative disorder in the elderly aged over 65. The extracellular accumulation of beta-amyloid (Aβ) aggregates in the brain is considered as the major event worsening the AD symptoms, but its underlying reason has remained unclear. Here the piezoelectric characteristics of Aβ aggregates are revealed. The vector piezoresponse force microscopy (PFM) analysis results exhibit that Aβ fibrils have spiraling piezoelectric domains along the length and a lateral piezoelectric constant of 44.1 pC N-1. Also, the continuous sideband Kelvin probe force microscopy (KPFM) images display that the increment of charge-induced surface potential on a single Aβ fibril is allowed to reach above +1700 mV in response to applied forces. These findings shed light on the peculiar mechano-electrical surface properties of pathological Aβ fibrils that exceed those of normal body components.

Thy1-ApoE4/C/EBPβ double transgenic mice act as a sporadic model with Alzheimer's disease

Early onset familial Alzheimer's disease (FAD) with APP, PS1/2 (presenilins) mutation accounts for only a small portion of AD cases, and most are late-onset sporadic. However, majority of AD mouse models are developed to mimic the genetic cause of human AD by overexpressing mutated forms of human APP, PS1/2, and/or Tau protein, though there is no Tau mutation in AD, and no single mouse model recapitulates all aspects of AD pathology. Here, we report Thy1-ApoE4/C/EBPβ double transgenic mouse model that demonstrates key AD pathologies in an age-dependent manner in absence of any human APP or PS1/2 mutation. Using the clinical diagnosis criteria, we show that this mouse model exhibits tempo-spatial features in AD patient brains, including progressive cognitive decline associated with brain atrophy, which is accompanied with extensive neuronal degeneration. Remarkably, the mice display gradual Aβ aggregation and neurofibrillary tangles formation in the brain validated by Aβ PET and Tau PET. Moreover, the mice reveal widespread neuroinflammation as shown in AD brains. Hence, Thy1-ApoE4/C/EBPβ mouse model acts as a sporadic AD mouse model, reconstituting the major AD pathologies.

The Kynurenine Pathway, Aryl Hydrocarbon Receptor, and Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of dementia, mainly affecting elderly individuals. AD is characterized by β-amyloid plaques, abnormal tau tangles, neuronal loss, and metabolic disruptions. Recent studies have revealed the involvement of the kynurenine (KP) pathway and the aryl hydrocarbon receptor (AhR) in AD development. The KP pathway metabolizes tryptophan to produce neuroactive substances like kynurenine, kynurenic acid, and quinolinic acid. In AD, high levels of kynurenine and the neurotoxic quinolinic acid are associated with increased neuroinflammation and excitotoxicity; conversely, reduced levels of kynurenic acid, which acts as a glutamate receptor antagonist, compromise neuroprotection. Research has indicated elevated KP metabolites and enzymes in the hippocampus of AD patients and other tissues such as blood, cerebrospinal fluid, and urine. However, the finding that KP metabolites are AD biomarkers in blood, cerebrospinal fluid, and urine has been controversial. This controversy, stemming from the lack of consideration of the specific stage of AD, details of the patient's treatment, cognitive deficits, and psychiatric comorbidities, underscores the need for more comprehensive research. AhR, a ligand-activated transcription factor, regulates immune response, oxidative stress, and xenobiotic metabolism. Various ligands, including tryptophan metabolites, can activate it. Some studies suggest that AhR activation contributes to AD, while others propose that it provides neuroprotection. This discrepancy may be explained by the specific ligands that activate AhR, highlighting the complex relationship between the KP pathway, AhR activation, and AD, where the same pathway can produce both neuroprotective and harmful effects.

Altered firing output of VIP interneurons and early dysfunctions in CA1 hippocampal circuits in the 3xTg mouse model of Alzheimer's disease

Alzheimer's disease (AD) leads to progressive memory decline, and alterations in hippocampal function are among the earliest pathological features observed in human and animal studies. GABAergic interneurons (INs) within the hippocampus coordinate network activity, among which type 3 interneuron-specific (I-S3) cells expressing vasoactive intestinal polypeptide and calretinin play a crucial role. These cells provide primarily disinhibition to principal excitatory cells (PCs) in the hippocampal CA1 region, regulating incoming inputs and memory formation. However, it remains unclear whether AD pathology induces changes in the activity of I-S3 cells, impacting the hippocampal network motifs. Here, using young adult 3xTg-AD mice, we found that while the density and morphology of I-S3 cells remain unaffected, there were significant changes in their firing output. Specifically, I-S3 cells displayed elongated action potentials and decreased firing rates, which was associated with a reduced inhibition of CA1 INs and their higher recruitment during spatial decision-making and object exploration tasks. Furthermore, the activation of CA1 PCs was also impacted, signifying early disruptions in CA1 network functionality. These findings suggest that altered firing patterns of I-S3 cells might initiate early-stage dysfunction in hippocampal CA1 circuits, potentially influencing the progression of AD pathology.

Impairment of brain function in a mouse model of Alzheimer's disease during the pre-depositing phase: The role of α7 nicotinic acetylcholine receptors

Alzheimer's disease (AD) is an age-dependent incurable neurodegenerative disorder accompanied by neuroinflammation, amyloid accumulation, and memory impairment. It begins decades before the first clinical symptoms appear, and identifying early biomarkers is key for developing disease-modifying therapies. We show now in a mouse model of AD that before any amyloid deposition the brains of 1.5-month-old mice contain increased levels of pro-inflammatory cytokines IL-1β and IL-6, decreased levels of nicotinic acetylcholine receptors (nAChRs) in the brain and brain mitochondria and increased amounts of α7 nAChR-bound Aβ1-42, along with impaired episodic memory and increased risk of apoptosis. Both acute (1-week-long) and chronic (4-month-long) treatments with α7-selective agonist PNU282987, starting at 1.5 months of age, were well tolerated. The acute treatment did not affect the levels of soluble Aβ1-42 but consistently upregulated the α7 nAChR expression, decreased the level of α7-Aβ1-42 complexes, and improved episodic memory of 1.5-month-old mice. The chronic treatment, covering the disease development phase, strongly upregulated the expression of all abundant brain nAChRs, reduced both free and α7-coupled Aβ1-42 within the brain, had anti-inflammatory and antiapoptotic effects, and potently upregulated cognition, thus identifying α7 nAChRs as both early biomarker and potent therapeutic target for fighting this devastating disease.

How do HCN channels play a part in Alzheimer's and Parkinson's disease?

Neurodegenerative diseases like Alzheimer's and Parkinson's disease (AD and PD) are well-known, yet their underlying causes remain unclear. Recent studies have suggested that disruption of ion channels contribute to their pathogenesis. Among these channels, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, encoded by HCN1-4 genes, are of particular interest due to their role in generating hyperpolarization-activated current (Ih), which is crucial in various neural activities impacting memory and motor functions. A growing body of evidence underscores the pivotal role of HCN in Aβ generation, glial cell function, and ischemia-induced dementia; while HCN is expressed in various regions of the basal ganglia, modulating their functions and influencing motor disorders in PD; neuroinflammation triggered by microglial activation represents a shared pathological mechanism in both AD and PD, in which HCN also plays a significant part. This review delves into the neuronal functions governed by HCN, its roles in the aforementioned pathogenesis, its expression patterns in AD and PD, and discusses potential therapeutic drugs targeting HCN for the treatment of these diseases, aiming to offer a novel perspective and inspire future research endeavors.

Microglia either promote or restrain TRAIL-mediated excitotoxicity caused by Aβ1-42 oligomers

BACKGROUND

Alzheimer's disease (AD) features progressive neurodegeneration and microglial activation that results in dementia and cognitive decline. The release of soluble amyloid (Aβ) oligomers into the extracellular space is an early feature of AD pathology. This can promote excitotoxicity and microglial activation. Microglia can adopt several activation states with various functional outcomes. Protective microglial activation states have been identified in response to Aβ plaque pathology in vivo. However, the role of microglia and immune mediators in neurotoxicity induced by soluble Aβ oligomers is unclear. Further, there remains a need to identify druggable molecular targets that promote protective microglial states to slow or prevent the progression of AD.

METHODS

Hippocampal entorhinal brain slice culture (HEBSC) was employed to study mechanisms of Aβ1-42 oligomer-induced neurotoxicity as well as the role of microglia. The roles of glutamate hyperexcitation and immune signaling in Aβ-induced neurotoxicity were assessed using MK801 and neutralizing antibodies to the TNF-related apoptosis-inducing ligand (TRAIL) respectively. Microglial activation state was manipulated using Gi-hM4di designer receptor exclusively activated by designer drugs (DREADDs), microglial depletion with the colony-stimulating factor 1 receptor (CSF1R) antagonist PLX3397, and microglial repopulation (PLX3397 withdrawal). Proteomic changes were assessed by LC-MS/MS in microglia isolated from control, repopulated, or Aβ-treated HEBSCs.

RESULTS

Neurotoxicity induced by soluble Aβ1-42 oligomers involves glutamatergic hyperexcitation caused by the proinflammatory mediator and death receptor ligand TRAIL. Microglia were found to have the ability to both promote and restrain Aβ-induced toxicity. Induction of microglial Gi-signaling with hM4di to prevent pro-inflammatory activation blunted Aβ neurotoxicity, while microglial depletion with CSF1R antagonism worsened neurotoxicity caused by Aβ as well as TRAIL. HEBSCs with repopulated microglia, however, showed a near complete resistance to Aβ-induced neurotoxicity. Comparison of microglial proteomes revealed that repopulated microglia have a baseline anti-inflammatory and trophic phenotype with a predicted pathway activation that is nearly opposite that of Aβ-exposed microglia. mTORC2 and IRF7 were identified as potential targets for intervention.

CONCLUSION

Microglia are key mediators of both protection and neurodegeneration in response to Aβ. Polarizing microglia toward a protective state could be used as a preventative strategy against Aβ-induced neurotoxicity.

Strategies to dissect microglia-synaptic interactions during aging and in Alzheimer's disease

Age is the largest risk factor for developing Alzheimer's disease (AD), a neurodegenerative disorder that causes a progressive and severe dementia. The underlying cause of cognitive deficits seen in AD is thought to be the disconnection of neural circuits that control memory and executive functions. Insight into the mechanisms by which AD diverges from normal aging will require identifying precisely which cellular events are driven by aging and which are impacted by AD-related pathologies. Since microglia, the brain-resident macrophages, are known to have critical roles in the formation and maintenance of neural circuits through synaptic pruning, they are well-positioned to modulate synaptic connectivity in circuits sensitive to aging or AD. In this review, we provide an overview of the current state of the field and on emerging technologies being employed to elucidate microglia-synaptic interactions in aging and AD. We also discuss the importance of leveraging genetic diversity to study how these interactions are shaped across more realistic contexts. We propose that these approaches will be essential to define specific aging- and disease-relevant trajectories for more personalized therapeutics aimed at reducing the effects of age or AD pathologies on the brain. This article is part of the Special Issue on "Microglia".

Dendritic spine head diameter predicts episodic memory performance in older adults

Episodic memory in older adults is varied and perceived to rely on numbers of synapses or dendritic spines. We analyzed 2157 neurons among 128 older individuals from the Religious Orders Study and Rush Memory and Aging Project. Analysis of 55,521 individual dendritic spines by least absolute shrinkage and selection operator regression and nested model cross-validation revealed that the dendritic spine head diameter in the temporal cortex, but not the premotor cortex, improved the prediction of episodic memory performance in models containing β amyloid plaque scores, neurofibrillary tangle pathology, and sex. These findings support the emerging hypothesis that, in the temporal cortex, synapse strength is more critical than quantity for memory in old age.

β-amyloid monomer scavenging by an anticalin protein prevents neuronal hyperactivity in mouse models of Alzheimer's Disease

Hyperactivity mediated by synaptotoxic β-amyloid (Aβ) oligomers is one of the earliest forms of neuronal dysfunction in Alzheimer's disease. In the search for a preventive treatment strategy, we tested the effect of scavenging Aβ peptides before Aβ plaque formation. Using in vivo two-photon calcium imaging and SF-iGluSnFR-based glutamate imaging in hippocampal slices, we demonstrate that an Aβ binding anticalin protein (Aβ-anticalin) can suppress early neuronal hyperactivity and synaptic glutamate accumulation in the APP23xPS45 mouse model of β-amyloidosis. Our results suggest that the sole targeting of Aβ monomers is sufficient for the hyperactivity-suppressing effect of the Aβ-anticalin at early disease stages. Biochemical and neurophysiological analyses indicate that the Aβ-anticalin-dependent depletion of naturally secreted Aβ monomers interrupts their aggregation to neurotoxic oligomers and, thereby, reverses early neuronal and synaptic dysfunctions. Thus, our results suggest that Aβ monomer scavenging plays a key role in the repair of neuronal function at early stages of AD.

Tau depletion in human neurons mitigates Aβ-driven toxicity

Alzheimer's disease (AD) is an age-related neurodegenerative condition and the most common type of dementia, characterised by pathological accumulation of extracellular plaques and intracellular neurofibrillary tangles that mainly consist of amyloid-β (Aβ) and hyperphosphorylated tau aggregates, respectively. Previous studies in mouse models with a targeted knock-out of the microtubule-associated protein tau (Mapt) gene demonstrated that Aβ-driven toxicity is tau-dependent. However, human cellular models with chronic tau lowering remain unexplored. In this study, we generated stable tau-depleted human induced pluripotent stem cell (iPSC) isogenic panels from two healthy individuals using CRISPR-Cas9 technology. We then differentiated these iPSCs into cortical neurons in vitro in co-culture with primary rat cortical astrocytes before conducting electrophysiological and imaging experiments for a wide range of disease-relevant phenotypes. Both AD brain derived and recombinant Aβ were used in this study to elicit toxic responses from the iPSC-derived cortical neurons. We showed that tau depletion in human iPSC-derived cortical neurons caused considerable reductions in neuronal activity without affecting synaptic density. We also observed neurite outgrowth impairments in two of the tau-depleted lines used. Finally, tau depletion protected neurons from adverse effects by mitigating the impact of exogenous Aβ-induced hyperactivity, deficits in retrograde axonal transport of mitochondria, and neurodegeneration. Our study established stable human iPSC isogenic panels with chronic tau depletion from two healthy individuals. Cortical neurons derived from these iPSC lines showed that tau is essential in Aβ-driven hyperactivity, axonal transport deficits, and neurodegeneration, consistent with studies conducted in Mapt-/- mouse models. These findings highlight the protective effects of chronic tau lowering strategies in AD pathogenesis and reinforce the potential in clinical settings. The tau-depleted human iPSC models can now be applied at scale to investigate the involvement of tau in disease-relevant pathways and cell types.

Selective suppression of oligodendrocyte-derived amyloid beta rescues neuronal dysfunction in Alzheimer's disease

Reduction of amyloid beta (Aβ) has been shown to be effective in treating Alzheimer's disease (AD), but the underlying assumption that neurons are the main source of pathogenic Aβ is untested. Here, we challenge this prevailing belief by demonstrating that oligodendrocytes are an important source of Aβ in the human brain and play a key role in promoting abnormal neuronal hyperactivity in an AD knock-in mouse model. We show that selectively suppressing oligodendrocyte Aβ production improves AD brain pathology and restores neuronal function in the mouse model in vivo. Our findings suggest that targeting oligodendrocyte Aβ production could be a promising therapeutic strategy for treating AD.

Transcranial alternating current stimulation (tACS) at gamma frequency: an up-and-coming tool to modify the progression of Alzheimer's Disease

The last decades have witnessed huge efforts devoted to deciphering the pathological mechanisms underlying Alzheimer's Disease (AD) and to testing new drugs, with the recent FDA approval of two anti-amyloid monoclonal antibodies for AD treatment. Beyond these drug-based experimentations, a number of pre-clinical and clinical trials are exploring the benefits of alternative treatments, such as non-invasive stimulation techniques on AD neuropathology and symptoms. Among the different non-invasive brain stimulation approaches, transcranial alternating current stimulation (tACS) is gaining particular attention due to its ability to externally control gamma oscillations. Here, we outline the current knowledge concerning the clinical efficacy, safety, ease-of-use and cost-effectiveness of tACS on early and advanced AD, applied specifically at 40 Hz frequency, and also summarise pre-clinical results on validated models of AD and ongoing patient-centred trials.

ZCCHC17 knockdown phenocopies Alzheimer's disease-related loss of synaptic proteins and hyperexcitability

ZCCHC17 is a master regulator of synaptic gene expression and has recently been shown to play a role in splicing of neuronal mRNA. We previously showed that ZCCHC17 protein declines in Alzheimer's disease (AD) brain tissue before there is significant gliosis and neuronal loss, that ZCCHC17 loss partially replicates observed splicing abnormalities in AD brain tissue, and that maintenance of ZCCHC17 levels is predicted to support cognitive resilience in AD. Here, we assessed the functional consequences of reduced ZCCHC17 expression in primary cortical neuronal cultures using siRNA knockdown. Consistent with its previously identified role in synaptic gene expression, loss of ZCCHC17 led to loss of synaptic protein expression. Patch recording of neurons shows that ZCCHC17 loss significantly disrupted the excitation/inhibition balance of neurotransmission, and favored excitatory-dominant synaptic activity as measured by an increase in spontaneous excitatory post synaptic currents and action potential firing rate, and a decrease in spontaneous inhibitory post synaptic currents. These findings are consistent with the hyperexcitable phenotype seen in AD animal models and in patients. We are the first to assess the functional consequences of ZCCHC17 knockdown in neurons and conclude that ZCCHC17 loss partially phenocopies AD-related loss of synaptic proteins and hyperexcitability.

Disruption in functional networks mediated tau spreading in Alzheimer's disease

Alzheimer's disease may be conceptualized as a 'disconnection syndrome', characterized by the breakdown of neural connectivity within the brain as a result of amyloid-beta plaques, tau neurofibrillary tangles and other factors leading to progressive degeneration and shrinkage of neurons, along with synaptic dysfunction. It has been suggested that misfolded tau proteins spread through functional connections (known as 'prion-like' properties of tau). However, the local effect of tau spreading on the synaptic function and communication between regions is not well understood. I aimed to investigate how the spreading of tau aggregates through connections can locally influence functional connectivity. In total, the imaging data of 211 participants including 117 amyloid-beta-negative non-demented and 94 amyloid-beta-positive non-demented participants were recruited from the Alzheimer's Disease Neuroimaging Initiative. Furthermore, normative resting-state functional MRI connectomes were used to model tau spreading through functional connections, and functional MRI of the included participants was used to determine the effect of tau spreading on functional connectivity. I found that lower functional connectivity to tau epicentres is associated with tau spreading through functional connections in both amyloid-beta-negative and amyloid-beta-positive participants. Also, amyloid-beta-PET in tau epicentres mediated the association of tau spreading and functional connectivity to epicentres suggesting a partial mediating effect of amyloid-beta deposition in tau epicentres on the local effect of tau spreading on functional connectivity. My findings provide strong support for the notion that tau spreading through connection is locally associated with disrupted functional connectivity between tau epicentre and non-epicentre regions independent of amyloid-beta pathology. Also, I defined several groups based on the relationship between tau spreading and functional disconnection, which provides quantitative assessment to investigate susceptibility or resilience to functional disconnection related to tau spreading. I showed that amyloid-beta, other copathologies and the apolipoprotein E epsilon 4 allele can be a leading factor towards vulnerability to tau relative functional disconnection.

Endogenous tau released from human ReNCell VM cultures by neuronal activity is phosphorylated at multiple sites

Tau is an intracellular protein but also known to be released into the extracellular fluid. Tau release mechanisms have drawn intense attention as these are known to play a key role in Alzheimer's disease (AD) pathology. However, tau can also be released under physiological conditions although its physiological function and release mechanisms have been poorly characterized, especially in human neuronal cells. We investigated endogenous tau release in ReNCell VM, a human neuroprogenitor cell line, under physiological conditions and found that tau is spontaneously released from cells. To study activity-dependent release of endogenous tau, human ReNCell VM culture was stimulated by 100μM AMPA or 50mM KCl for one-hour, tau was actively released to the culture medium. The released tau was highly phosphorylated at nine phosphorylation sites (pSites) detected by phospho-specific tau antibodies including AT270 (T175/T181), AT8 (S202/T205), AT100 (T212/S214), AT180 (T231), and PHF-1 (S396/S404), showing that these pSites are important for activity-dependent tau release from human ReNCell VM. Intracellular tau showed various phosphorylation status across these sites, with AT270 and PHF-1 highly phosphorylated while AT8 and AT180 were minimally phosphorylated, suggesting that AT8 and AT180 pSites exhibit a propensity for secretion rather than being retained intracellularly. This activity-dependent tau release was significantly decreased by inhibition of GSK-3β, demonstrating that GSK3β-dependent phosphorylation of tau plays an important role in its release by neuronal activity. In this study, we showed that ReNCell VM serves as a valuable model for studying endogenous physiological tau release. Further, ReNCell model can be also used to study pathological release of human tau that will contribute to our understanding of the progression of AD and related dementias.