Different electrophysiological actions of 24- and 72-hour aggregated amyloid-beta oligomers on hippocampal field population spike in both anesthetized and awake rats

Nanomechanical Profiling of Aβ42 Oligomer-Induced Biological Changes in Single Hippocampus Neurons

Understanding how Aβ42 oligomers induce changes in neurons from a mechanobiological perspective has important implications in neuronal dysfunction relevant to neurodegenerative diseases. However, it remains challenging to profile the mechanical responses of neurons and correlate the mechanical signatures to the biological properties of neurons given the structural complexity of cells. Here, we quantitatively investigate the nanomechanical properties of primary hippocampus neurons upon exposure to Aβ42 oligomers at the single neuron level by using atomic force microscopy (AFM). We develop a method termed heterogeneity-load-unload nanomechanics (HLUN), which exploits the AFM force spectra in the whole loading-unloading cycle, allowing comprehensive profiling of the mechanical properties of living neurons. We extract four key nanomechanical parameters, including the apparent Young's modulus, cell spring constant, normalized hysteresis, and adhesion work, that serve as the nanomechanical signatures of neurons treated with Aβ42 oligomers. These parameters are well-correlated with neuronal height increase, cortical actin filament strengthening, and calcium concentration elevation. Thus, we establish an HLUN method-based AFM nanomechanical analysis tool for single neuron study and build an effective correlation between the nanomechanical profile of the single neurons and the biological effects triggered by Aβ42 oligomers. Our finding provides useful information on the dysfunction of neurons from the mechanobiological perspective.

Barbeau E, Pariente J, Dahan L, Valton L. Sleep: The Tip of the Iceberg in the Bidirectional Link Between Alzheimer's Disease and Epilepsy

The observation that a pathophysiological link might exist between Alzheimer's disease (AD) and epilepsy dates back to the identification of the first cases of the pathology itself and is now strongly supported by an ever-increasing mountain of literature. An overwhelming majority of data suggests not only a higher prevalence of epilepsy in Alzheimer's disease compared to healthy aging, but also that AD patients with a comorbid epileptic syndrome, even subclinical, have a steeper cognitive decline. Moreover, clinical and preclinical investigations have revealed a marked sleep-related increase in the frequency of epileptic activities. This characteristic might provide clues to the pathophysiological pathways underlying this comorbidity. Furthermore, the preferential sleep-related occurrence of epileptic events opens up the possibility that they might hasten cognitive decline by interfering with the delicately orchestrated synchrony of oscillatory activities implicated in sleep-related memory consolidation. Therefore, we scrutinized the literature for mechanisms that might promote sleep-related epileptic activity in AD and, possibly dementia onset in epilepsy, and we also aimed to determine to what degree and through which processes such events might alter the progression of AD. Finally, we discuss the implications for patient care and try to identify a common basis for methodological considerations for future research and clinical practice.

Amyloid-β: a potential link between epilepsy and cognitive decline

People with epilepsy - in particular, late-onset epilepsy of unknown aetiology - have an elevated risk of dementia, and seizures have been detected in the early stages of Alzheimer disease (AD), supporting the concept of an epileptic AD prodrome. However, the relationship between epilepsy and cognitive decline remains controversial, with substantial uncertainties about whether epilepsy drives cognitive decline or vice versa, and whether shared pathways underlie both conditions. Here, we review evidence that amyloid-β (Aβ) forms part of a shared pathway between epilepsy and cognitive decline, particularly in the context of AD. People with epilepsy show an increased burden of Aβ pathology in the brain, and Aβ-mediated epileptogenic alterations have been demonstrated in experimental studies, with evidence suggesting that Aβ pathology might already be pro-epileptogenic at the soluble stage, long before plaque deposition. We discuss the hypothesis that Aβ mediates - or is at least a major determinant of - a continuum spanning epilepsy and cognitive decline. Serial cognitive testing and assessment of Aβ levels might be worthwhile to stratify the risk of developing dementia in people with late-onset epilepsy. If seizures are a clinical harbinger of dementia, people with late-onset epilepsy could be an ideal group in which to implement preventive or therapeutic strategies to slow cognitive decline.

Firing rate homeostasis counteracts changes in stability of recurrent neural networks caused by synapse loss in Alzheimer's disease

The impairment of cognitive function in Alzheimer's disease is clearly correlated to synapse loss. However, the mechanisms underlying this correlation are only poorly understood. Here, we investigate how the loss of excitatory synapses in sparsely connected random networks of spiking excitatory and inhibitory neurons alters their dynamical characteristics. Beyond the effects on the activity statistics, we find that the loss of excitatory synapses on excitatory neurons reduces the network's sensitivity to small perturbations. This decrease in sensitivity can be considered as an indication of a reduction of computational capacity. A full recovery of the network's dynamical characteristics and sensitivity can be achieved by firing rate homeostasis, here implemented by an up-scaling of the remaining excitatory-excitatory synapses. Mean-field analysis reveals that the stability of the linearised network dynamics is, in good approximation, uniquely determined by the firing rate, and thereby explains why firing rate homeostasis preserves not only the firing rate but also the network's sensitivity to small perturbations.

Sleep is bi-directionally modified by amyloid beta oligomers

Disrupted sleep is a major feature of Alzheimer’s disease (AD), often arising years before symptoms of cognitive decline. Prolonged wakefulness exacerbates the production of amyloid-beta (Aβ) species, a major driver of AD progression, suggesting that sleep loss further accelerates AD through a vicious cycle. However, the mechanisms by which Aβ affects sleep are unknown. We demonstrate in zebrafish that Aβ acutely and reversibly enhances or suppresses sleep as a function of oligomer length. Genetic disruptions revealed that short Aβ oligomers induce acute wakefulness through Adrenergic receptor b2 (Adrb2) and Progesterone membrane receptor component 1 (Pgrmc1), while longer Aβ forms induce sleep through a pharmacologically tractable Prion Protein (PrP) signaling cascade. Our data indicate that Aβ can trigger a bi-directional sleep/wake switch. Alterations to the brain’s Aβ oligomeric milieu, such as during the progression of AD, may therefore disrupt sleep via changes in acute signaling events.

Early-Onset Network Hyperexcitability in Presymptomatic Alzheimer's Disease Transgenic Mice Is Suppressed by Passive Immunization with Anti-Human APP/Aβ Antibody and by mGluR5 Blockade

Cortical and hippocampal network hyperexcitability appears to be an early event in Alzheimer’s disease (AD) pathogenesis, and may contribute to memory impairment. It remains unclear if network hyperexcitability precedes memory impairment in mouse models of AD and what are the underlying cellular mechanisms. We thus evaluated seizure susceptibility and hippocampal network hyperexcitability at ~3 weeks of age [prior to amyloid beta (Aβ) plaque deposition, neurofibrillary pathology, and cognitive impairment] in a triple transgenic mouse model of familial AD (3xTg-AD mouse) that harbors mutated human Aβ precursor protein (APP), tau and presenilin 1 (PS1) genes. Audiogenic seizures were elicited in a higher proportion of 3xTg-AD mice compared with wild type (WT) controls. Seizure susceptibility in 3xTg-AD mice was attenuated either by passive immunization with anti-human APP/Aβ antibody (6E10) or by blockade of metabotropic glutamate receptor 5 (mGluR5) with the selective antagonist, 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). In in vitro hippocampal slices, suppression of synaptic inhibition with the GABA_A receptor antagonist, bicuculline, induced prolonged epileptiform (>1.5 s in duration) ictal-like discharges in the CA3 neuronal network in the majority of the slices from 3xTg-AD mice. In contrast, only short epileptiform (<1.5 s in duration) interictal-like discharges were observed following bicuculline application in the CA3 region of WT slices. The ictal-like activity in CA3 region of the hippocampus was significantly reduced in the 6E10-immunized compared to the saline-treated 3xTg-AD mice. MPEP acutely suppressed the ictal-like discharges in 3xTg-AD slices. Remarkably, epileptiform discharge duration positively correlated with intraneuronal human (transgenic) APP/Aβ expression in the CA3 region of the hippocampus. Our data suggest that in a mouse model of familial AD, hypersynchronous network activity underlying seizure susceptibility precedes Aβ plaque pathology and memory impairment. This early-onset network hyperexcitability can be suppressed by passive immunization with an anti-human APP/Aβ antibody and by mGluR5 blockade in 3xTg-AD mice.

Davunetide improves spatial learning and memory in Alzheimer's disease-associated rats

Memory loss and cognition decline are the main clinical manifestations of Alzheimer's disease (AD). Amyloid β protein (Aβ) aggregated in the brain is one of the key pathological characteristics of AD and responsible for the deficits in learning and memory. It is reported that davunetide, an octapeptide derived from activity-dependent neuroprotective protein (ADNP), inhibited Aβ aggregation and Aβ-induced neurotoxicity. To further characterize the neuroprotective roles of davunetide and its possible mechanism, the present study investigated the effects of davunetide on Aβ1-42-induced impairments in spatial memory, synaptic plasticity and hippocampal AKT level. In Morris water maze (MWM) test, bilateral intrahippocampal injection of Aβ1-42 significantly increased escape latency and decreased target quadrant swimming time of rats, while three weeks of intranasal application of davunetide reversed the Aβ1-42-induced learning deficits and memory loss in a dose-dependent manner. In vivo field potentiation recording showed that Aβ1-42 suppressed long-term potentiation (LTP) of excitatory postsynaptic potential (fEPSP) in the hippocampal CA1 region of rats, while davunetide effectively blocked the suppression of LTP, without affecting paired-pulse facilitation (PPF). Western blotting experiments showed a significant decrease in the level of hippocampal p-AKT (Ser473), not total AKT, in Aβ1-42 only group, which was mostly antagonized by davunetide treatment. These findings demonstrate that davunetide, probably by enhancing PI3K/AKT pathway, plays an important positive role in attenuating Aβ1-42-induced impairments in spatial memory and synaptic plasticity, suggesting that davunetide could be an effective therapeutic candidate for the prevention and treatment of neurodegenerative disease such as AD.

Soluble Aβ oligomers impair hippocampal LTP by disrupting glutamatergic/GABAergic balance

Epileptic activity may be more prevalent in early stage Alzheimer's disease (AD) than previously believed. Several studies report spontaneous seizures and interictal discharges in mouse models of AD undergoing age-related Aβ accumulation. The mechanism by which Aβ-induced neuronal excitability can trigger epileptiform activity remains unknown. Here, we systematically examined field excitatory postsynaptic potentials (fEPSP) in stratum radiatum and population spikes (PSs) in the adjacent stratum pyramidale of CA1 in wild-type mouse hippocampal slices. Soluble Aβ oligomers (oAβ) blocked hippocampal LTP and EPSP-spike (E-S) potentiation, and these effects were occluded by prior treatment with the glutamate uptake inhibitor TBOA. In accord, oAβ elevated glutamate levels in the hippocampal slice medium. Recording the PS revealed that oAβ increased PS frequency and reduced LTP, and this LTP deficit was occluded by pretreatment with the GABAA antagonist picrotoxin. Whole-cell recordings showed that oAβ significantly increased spontaneous EPSC frequency. Decreasing neuronal activity by increasing GABA tone or partially blocking NMDAR activity prevented oAβ impairment of hippocampal LTP. Finally, treating slices with two antiepileptic drugs rescued the LTP inhibition induced by oAβ. We conclude that soluble Aβ oligomers at the low nanomolar levels present in AD brain increase neuronal excitability by disrupting glutamatergic/GABAergic balance, thereby impairing synaptic plasticity.

Alterations in CA1 pyramidal neuronal intrinsic excitability mediated by Ih channel currents in a rat model of amyloid beta pathology

Amyloid beta (Aβ) accumulation plays an important role in the pathogenesis of Alzheimer's disease (AD) by changing the neuronal excitability. However, the cellular mechanisms by which accumulation of Aβ affects intrinsic neuronal properties are not well understood. The effect of bilateral intra-frontal cortex Aβ (1-42) peptide injection on the intrinsic excitability of hippocampal CA1 pyramidal neurons with particular focus on the contribution of hyperpolarization-activated (Ih) channel currents was examined using whole-cell patch-clamp recording. Passive avoidance memory impairment and morphological changes in rats receiving intra-frontal Aβ treatment were observed, which was associated with significant changes both in passive and active intrinsic electrical membrane properties of CA1 pyramidal neurons. Electrophysiological recording showed a significant decrease in neuronal excitability associated with an augmentation in the first spike after-hyperpolarization (AHP) amplitude. In addition, the depolarizing sag voltage was altered in neurons recorded from Aβ-treated group. In voltage-clamp condition, a hyperpolarizing activated inward current sensitive to ZD7288 and capsaicin was significantly increased in neurons from Aβ-treated rats. The Ih current density was increased and the activation curve was shifted toward less negative potential in the Aβ-treated group as compared to control group. The enhancing effect of Aβ treatment on Ih current was confirmed by showing upregulation of the mRNA of HCN1 channel in the CA1 pyramidal layer of hippocampi. These findings suggest the contribution of Ih and possibly TRPV1 channel currents to the changes induced by Aβ treatment in the intrinsic membrane properties, which, in turn, may provide therapeutic targets for treatment of AD.

Accurate secondary structure prediction and fold recognition for circular dichroism spectroscopy

Circular dichroism (CD) spectroscopy is a widely used technique for the study of protein structure. Numerous algorithms have been developed for the estimation of the secondary structure composition from the CD spectra. These methods often fail to provide acceptable results on α/β-mixed or β-structure-rich proteins. The problem arises from the spectral diversity of β-structures, which has hitherto been considered as an intrinsic limitation of the technique. The predictions are less reliable for proteins of unusual β-structures such as membrane proteins, protein aggregates, and amyloid fibrils. Here, we show that the parallel/antiparallel orientation and the twisting of the β-sheets account for the observed spectral diversity. We have developed a method called β-structure selection (BeStSel) for the secondary structure estimation that takes into account the twist of β-structures. This method can reliably distinguish parallel and antiparallel β-sheets and accurately estimates the secondary structure for a broad range of proteins. Moreover, the secondary structure components applied by the method are characteristic to the protein fold, and thus the fold can be predicted to the level of topology in the CATH classification from a single CD spectrum. By constructing a web server, we offer a general tool for a quick and reliable structure analysis using conventional CD or synchrotron radiation CD (SRCD) spectroscopy for the protein science research community. The method is especially useful when X-ray or NMR techniques fail. Using BeStSel on data collected by SRCD spectroscopy, we investigated the structure of amyloid fibrils of various disease-related proteins and peptides.

Early onset of hypersynchronous network activity and expression of a marker of chronic seizures in the Tg2576 mouse model of Alzheimer's disease

Cortical and hippocampal hypersynchrony of neuronal networks seems to be an early event in Alzheimer’s disease pathogenesis. Many mouse models of the disease also present neuronal network hypersynchrony, as evidenced by higher susceptibility to pharmacologically-induced seizures, electroencephalographic seizures accompanied by spontaneous interictal spikes and expression of markers of chronic seizures such as neuropeptide Y ectopic expression in mossy fibers. This network hypersynchrony is thought to contribute to memory deficits, but whether it precedes the onset of memory deficits or not in mouse models remains unknown. The earliest memory impairments in the Tg2576 mouse model of Alzheimer’s disease have been observed at 3 months of age. We thus assessed network hypersynchrony in Tg2576 and non-transgenic male mice at 1.5, 3 and 6 months of age. As soon as 1.5 months of age, Tg2576 mice presented higher seizure susceptibility to systemic injection of a GABA_A receptor antagonist. They also displayed spontaneous interictal spikes on EEG recordings. Some Tg2576 mice presented hippocampal ectopic expression of neuropeptide Y which incidence seems to increase with age among the Tg2576 population. Our data reveal that network hypersynchrony appears very early in Tg2576 mice, before any demonstrated memory impairments.

Gastrodin suppresses the amyloid β-induced increase of spontaneous discharge in the entorhinal cortex of rats

Accumulated soluble amyloid beta- (Aβ-) induced aberrant neuronal network activity may directly contribute to cognitive deficits, which are the most outstanding characteristics of Alzheimer's disease (AD). The entorhinal cortex (EC) is one of the earliest affected brain regions in AD. Impairments of EC neurons are responsible for the cognitive deficits in AD. However, little effort has been made to investigate the effects of soluble Aβ on the discharge properties of EC neurons in vivo. The present study was designed to examine the effects of soluble Aβ₁₋₄₂ on the discharge properties of EC neurons, using in vivo extracellular single unit recordings. The protective effects of gastrodin (GAS) were also investigated against Aβ₁₋₄₂-induced alterations in EC neuronal activities. The results showed that the spontaneous discharge of EC neurons was increased by local application of soluble Aβ₁₋₄₂ and that GAS can effectively reverse Aβ₁₋₄₂-induced facilitation of spontaneous discharge in a concentration-dependent manner. Moreover, whole-cell patch clamp results indicated that the protective function of GAS on abnormal hyperexcitability may be partially mediated by its inhibitory action on Aβ₁₋₄₂-elicited inward currents in EC neurons. Our study suggested that GAS may provide neuroprotective effects on Aβ₁₋₄₂-induced hyperactivity in EC neurons of rats.

Down syndrome and dementia: seizures and cognitive decline

The objective of this study was to determine the association of seizures and cognitive decline in adults with Down syndrome (DS) and Alzheimer's-type dementia. A retrospective data analysis was carried out following a controlled study of antioxidant supplementation for dementia in DS. Observations were made at baseline and every 6 months for 2 years. Seizure history was obtained from study records. The primary outcome measures comprised the performance-based Severe Impairment Battery (SIB) and Brief Praxis Test (BPT). Secondary outcome measures comprised the informant-based Dementia Questionnaire for Mentally Retarded Persons and Vineland Adaptive Behavior Scales. Because a large proportion of patients with seizures had such severe cognitive decline as to become untestable on the performance measures, time to "first inability to test" was measured. Adjustments were made for the potentially confounding co-variates of age, gender, APOE4 status, baseline cognitive impairment, years since dementia onset at baseline, and treatment assignment. The estimated odds ratio for the time to "first inability to test" on SIB comparing those with seizures to those without is 11.02 (95% CI: 1.59, 76.27), a ratio that is significantly different from 1 (p = 0.015). Similarly, we estimated an odds ratio of 9.02 (95% CI: 1.90, 42.85) on BPT, a ratio also significantly different than 1 (p = 0.006). Results from a secondary analysis of the informant measures showed significant decline related to seizures. We conclude that there is a strong association of seizures with cognitive decline in demented individuals with DS. Prospective studies exploring this relationship in DS are indicated.

Mechanisms of epileptogenesis and potential treatment targets

Prevention of epileptogenesis after brain trauma is an unmet medical challenge. Recent molecular profiling studies have provided an insight into molecular changes that contribute to formation of ictogenic neuronal networks, including genes regulating synaptic or neuronal plasticity, cell death, proliferation, and inflammatory or immune responses. These mechanisms have been targeted to prevent epileptogenesis in animal models. Favourable effects have been obtained using immunosuppressants, antibodies blocking adhesion of leucocytes to endothelial cells, gene therapy driving expression of neurotrophic factors, pharmacological neurostimulation, or even with conventional antiepileptic drugs by administering them before the appearance of genetic epilepsy. Further studies are needed to clarify the optimum time window and aetiological specificity of treatments. Questions related to adverse events also need further consideration. Encouragingly, the recent experimental studies emphasise that the complicated process of epileptogenesis can be favourably modified, and that antiepileptogenesis as a treatment indication might not be an impossible mission.