Longitudinal excitation-inhibition balance altered by sex and APOE-ε4

Neuronal hyperexcitation affects memory and neural processing across the Alzheimer's disease (AD) cognitive continuum. Levetiracetam, an antiepileptic, shows promise in improving cognitive impairment by restoring the neural excitation/inhibition balance in AD patients. We previously identified a hyper-excitable phenotype in cognitively unimpaired female APOE-ε4 carriers relative to male counterparts cross-sectionally. This sex difference lacks longitudinal validation; however, clarifying the vulnerability of female ε4-carriers could better inform antiepileptic treatment efficacy. Here, we investigated this sex-by-ε4 interaction using a longitudinal design. We used resting-state fMRI and diffusion tensor imaging collected longitudinally from 106 participants who were cognitively unimpaired for at least one scan event but may have been assessed to have clinical dementia ratings corresponding to early mild cognitive impairment over time. By including scan events where participants transitioned to mild cognitive impairment, we modeled the trajectory of the whole-brain excitation-inhibition ratio throughout the preclinical cognitively healthy continuum and extended to early impairment. A linear mixed model revealed a significant three-way interaction among sex, ε4-status, and time, with female ε4-carriers showing a significant hyper-excitable trajectory. These findings suggest a possible pathway for preventative therapy targeting preclinical hyperexcitation in female ε4-carriers.

Pathological tau alters head direction signaling and induces spatial disorientation

Spatial disorientation, an early symptom of dementia, is emerging as an early and reliable cognitive biomarker predicting future memory problems associated with Alzheimer's disease, but the underlying neural mechanisms have yet to be fully defined. The anterodorsal thalamic nucleus (ADn) exhibits early and selective vulnerability to pathological misfolded forms of tau, a major hallmark of Alzheimer's disease and ageing. The ADn contains a high density of head direction (HD) cells, which contribute to spatial navigation and orientation. Hence, their disruption may contribute to spatial disorientation. To test this, we virally expressed human mutant tau (htau) in the ADn of adult mice. HD-tau mice were defined by phosphorylated and oligomeric forms of htau in ADn somata and in axon terminals in postsynaptic target regions. Compared to controls, HD-tau mice exhibited increased looping behavior during spatial learning, and made a greater number of head turns during memory recall, indicative of spatial disorientation. Using in vivo extracellular recordings, we identified htau-expressing ADn cells and found a lower proportion of HD cells in the ADn from HD-tau mice, along with reduced directionality and altered burst firing. These findings provide evidence that expression of pathological human tau can alter HD signaling, leading to impairments in spatial orientation.

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.

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

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

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.

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

Background

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

Methods

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

Results

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

Conclusions

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

Supplementary Information

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

Altered excitatory and inhibitory neuronal subpopulation parameters are distinctly associated with tau and amyloid in Alzheimer's disease

Background

Neuronal- and circuit-level abnormalities of excitation and inhibition are shown to be associated with tau and amyloid-beta (Aβ) in preclinical models of Alzheimer's disease (AD). These relationships remain poorly understood in patients with AD.

Methods

Using empirical spectra from magnetoencephalography and computational modeling (neural mass model), we examined excitatory and inhibitory parameters of neuronal subpopulations and investigated their specific associations to regional tau and Aβ, measured by positron emission tomography, in patients with AD.

Results

Patients with AD showed abnormal excitatory and inhibitory time-constants and neural gains compared to age-matched controls. Increased excitatory time-constants distinctly correlated with higher tau depositions while increased inhibitory time-constants distinctly correlated with higher Aβ depositions.

Conclusions

Our results provide critical insights about potential mechanistic links between abnormal neural oscillations and cellular correlates of impaired excitatory and inhibitory synaptic functions associated with tau and Aβ in patients with AD.

Funding

This study was supported by the National Institutes of Health grants: K08AG058749 (KGR), F32AG050434-01A1 (KGR), K23 AG038357 (KAV), P50 AG023501, P01 AG19724 (BLM), P50-AG023501 (BLM and GDR), R01 AG045611 (GDR); AG034570, AG062542 (WJ); NS100440 (SSN), DC176960 (SSN), DC017091 (SSN), AG062196 (SSN); a grant from John Douglas French Alzheimer's Foundation (KAV); grants from Larry L. Hillblom Foundation: 2015-A-034-FEL (KGR), 2019-A-013-SUP (KGR); grants from the Alzheimer's Association: AARG-21-849773 (KGR); PCTRB-13-288476 (KAV), and made possible by Part the CloudTM (ETAC-09-133596); a grant from Tau Consortium (GDR and WJJ), and a gift from the S. D. Bechtel Jr. Foundation.

The effect and mechanism of 19S proteasome PSMD11/Rpn6 subunit in D-Galactose induced mimetic aging models

Regulating proteasome activity is a potent therapeutic aspect of age-related hearing loss, which has been proven to protect neurons from age-related damaging. PSMD11, subunit of the 19S proteasome regulatory particle, is known to mainly up-regulate proteasome activity and prolong aging. However, the mechanism of PSMD11 in age-related hearing loss has not been deeply explored. In the present study, we explore the function and mechanism of PSMD11 protecting neurons in d-Galactose (D-Gal) mimetic aging models. Age-related pathologies were detected by Taq-PCR, ABR, Transmission electron microscopy, toluidine blue and β-galactosidase staining. The relative expressions of the proteins were explored by Western blotting, oxyblot, immunoprecipitation and immunofluorescence. Flow cytometry was used to manifest the oxidative state. We discovered that proteasome activity was impaired with aging, and that ROS and toxic protein accumulated in D-Gal induced aging models. PSMD11 changed with aging, and was associated with the metabolism of proteasome activity in the D-Gal treated models. Moreover, the knockdown or overexpression of PSMD11 was sufficient to change the oxidative state caused by D-Gal. Our results also demonstrated that PSMD11 could bond to AMPKα1/2 in the auditory cortex and PC12 cells, and AMPKα2 but not AMPKα1 was efficient to regulate the function of PSMD11. Deeper insights into the mechanisms of regulating PSMD11 for the anti-aging process are needed, and may offer novel therapeutic methods for central presbycusis.