Beta-amyloid peptide blocks the fast-inactivating K+ current in rat hippocampal neurons

Influence of amyloid beta on impulse spiking of isolated hippocampal neurons

One of the signs of Alzheimer's disease (AD) is the formation of β-amyloid plaques, which ultimately lead to the dysfunction of neurons with subsequent neurodegeneration. Although extensive researches have been conducted on the effects of different amyloid conformations such as oligomers and fibrils on neuronal function in isolated cells and circuits, the exact contribution of extracellular beta-amyloid on neurons remains incompletely comprehended. In our experiments, we studied the effect of β-amyloid peptide (Aβ1-42) on the action potential (APs) generation in isolated CA1 hippocampal neurons in perforated patch clamp conditions. Our findings demonstrate that Aβ1-42 affects the generation of APs differently in various hippocampal neurons, albeit with a shared effect of enhancing the firing response of the neurons within a minute of the start of Aβ1-42 application. In the first response type, there was a shift of 20-65% toward smaller values in the firing threshold of action potentials in response to inward current. Conversely, the firing threshold of action potentials was not affected in the second type of response to the application of Aβ1-42. In these neurons, Aβ1-42 caused a moderate increase in the frequency of spiking, up to 15%, with a relatively uniform increase in the frequency of action potentials generation regardless of the level of input current. Obtained data prove the absence of direct short-term negative effect of the Aβ1-42 on APs generation in neurons. Even with increasing the APs generation frequency and lowering the neurons' activation threshold, neurons were functional. Obtained data can suggest that only the long-acting presence of the Aβ1-42 in the cell environment can cause neuronal dysfunction due to a prolonged increase of APs firing and predisposition to this process.

Kisspeptin-13 prevented the electrophysiological alterations induced by Amyloid-Beta pathology in rat: Possible involvement of stromal interaction molecules and pCREB

Alzheimer's disease (AD) is a progressive neurological disease that slowly causing memory impairments with no effective treatment. We have recently reported that kisspeptin-13 (KP-13) ameliorates Aβ toxicity-induced memory deficit in rats. Here, the possible cellular impact of kisspeptin receptor activation in a rat model of the early stage AD was assessed using whole-cell patch-clamp recording from CA1 pyramidal neurons and molecular approaches. Compared to neurons from the control group, cells from the Aβ-treated group displayed spontaneous and evoked hyperexcitability with lower spike frequency adaptation. These cells had also a lower sag ratio in response to hyperpolarizing prepulse current delivered before a depolarizing current injection. Neurons from the Aβ-treated group exhibited short spike onset latency, lower rheobase and short utilization time compared with those in the control group. Furthermore, phase plot analysis of action potential showed that Aβ treatment affected the action potential features. These electrophysiological changes induced by Aβ were associated with increased expression of stromal interaction molecules (STIMs), particularly (STIM2) and decreased pCREB/CREB ratio. Treatment with KP-13 following Aβ injection into the entorhinal cortex, however, prevented the excitatory effect of Aβ on spontaneous and evoked neuronal activity, increased the latency of onset, enhanced the sag ratio, increased the rheobase and utilization time, and prevented the changes induced Aβ on spike parameters. In addition, the KP-13 application after Aβ treatment reduced the expression of STIMs and increased the pCREB/CREB ratio compared to those receiving Aβ treatment alone. In summary, these results provide evidence that activation of kisspeptin receptor may be effective against pathology of Aβ.

Subcellular localization of hippocampal ryanodine receptor 2 and its role in neuronal excitability and memory

Ryanodine receptor 2 (RyR2) is abundantly expressed in the heart and brain. Mutations in RyR2 are associated with both cardiac arrhythmias and intellectual disability. While the mechanisms of RyR2-linked arrhythmias are well characterized, little is known about the mechanism underlying RyR2-associated intellectual disability. Here, we employed a mouse model expressing a green fluorescent protein (GFP)-tagged RyR2 and a specific GFP probe to determine the subcellular localization of RyR2 in hippocampus. GFP-RyR2 was predominantly detected in the soma and dendrites, but not the dendritic spines of CA1 pyramidal neurons or dentate gyrus granular neurons. GFP-RyR2 was also detected within the mossy fibers in the stratum lucidum of CA3, but not in the presynaptic terminals of CA1 neurons. An arrhythmogenic RyR2-R4496C^+/− mutation downregulated the A-type K⁺ current and increased membrane excitability, but had little effect on the afterhyperpolarization current or presynaptic facilitation of CA1 neurons. The RyR2-R4496C^+/− mutation also impaired hippocampal long-term potentiation, learning, and memory. These data reveal the precise subcellular distribution of hippocampal RyR2 and its important role in neuronal excitability, learning, and memory.

A mouse model containing a GFP-tagged ryanodine receptor 2 (RyR2) has shed light on the precise subcellular localization of hippocampal RyR2 and mechanisms underlying neuronal excitability, learning, and memory.

Limiting RyR2 Open Time Prevents Alzheimer's Disease-Related Neuronal Hyperactivity and Memory Loss but Not β-Amyloid Accumulation

Neuronal hyperactivity is an early primary dysfunction in Alzheimer’s disease (AD) in humans and animal models, but effective neuronal hyperactivity-directed anti-AD therapeutic agents are lacking. Here we define a previously unknown mode of ryanodine receptor 2 (RyR2) control of neuronal hyperactivity and AD progression. We show that a single RyR2 point mutation, E4872Q, which reduces RyR2 open time, prevents hyperexcitability, hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset AD mouse model (5xFAD). The RyR2-E4872Q mutation upregulates hippocampal CA1-pyramidal cell A-type K⁺ current, a well-known neuronal excitability control that is downregulated in AD. Pharmacologically limiting RyR2 open time with the R-carvedilol enantiomer (but not racemic carvedilol) prevents and rescues neuronal hyperactivity, memory impairment, and neuron loss even in late stages of AD. These AD-related deficits are prevented even with continued β-amyloid accumulation. Thus, limiting RyR2 open time may be a hyperactivity-directed, non-β-amyloid-targeted anti-AD strategy.

Yao et al. show that genetically or pharmacologically limiting the open duration of ryanodine receptor 2 upregulates the A-type potassium current and prevents neuronal hyperexcitability and hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset Alzheimer’s disease mouse model, even with continued accumulation of β-amyloid.

Resveratrol targeting tau proteins, amyloid-beta aggregations, and their adverse effects: An updated review

Resveratrol (Res) is a non-flavonoid compound with pharmacological actions such as antioxidant, antiinflammatory, hepatoprotective, antidiabetes, and antitumor. This plant-derived chemical has a long history usage in treatment of diseases. The excellent therapeutic impacts of Res and its capability in penetration into blood-brain barrier have made it an appropriate candidate in the treatment of neurological disorders (NDs). Tau protein aggregations and amyloid-beta (Aβ) deposits are responsible for the induction of NDs. A variety of studies have elucidated the role of these aggregations in NDs and the underlying molecular pathways in their development. In the present review, based on the recently published articles, we describe that how Res administration could inhibit amyloidogenic pathway and stimulate processes such as autophagy to degrade Aβ aggregations. Besides, we demonstrate that Res supplementation is beneficial in dephosphorylation of tau proteins and suppressing their aggregations. Then, we discuss molecular pathways and relate them to the treatment of NDs.

CIRBP Ameliorates Neuronal Amyloid Toxicity via Antioxidative and Antiapoptotic Pathways in Primary Cortical Neurons

It is generally accepted that the amyloid β (Aβ) peptide toxicity contributes to neuronal loss and is involved in the initiation and progression of Alzheimer's disease (AD). Cold-inducible RNA-binding protein (CIRBP) is reported to be a general stress-response protein, which is induced by different stress conditions. Previous reports have shown the neuroprotective effects of CIRBP through the suppression of apoptosis via the Akt and ERK pathways. The objective of this study is to examine the effect of CIRBP against Aβ-induced toxicity in cultured rat primary cortical neurons and attempt to uncover its underlying mechanism. Here, MTT, LDH release, and TUNEL assays showed that CIRBP overexpression protected against both intracellular amyloid β- (iAβ-) induced and Aβ _25-35-induced cytotoxicity in rat primary cortical neurons. Electrophysiological changes responsible for iAβ-induced neuronal toxicity, including an increase in neuronal resting membrane potentials and a decrease in K⁺ currents, were reversed by CIRBP overexpression. Western blot results further showed that Aβ _25-35 treatment significantly increased the level of proapoptotic protein Bax, cleaved caspase-3, and cleaved caspase-9 and decreased the level of antiapoptotic factor Bcl-2, but were rescued by CIRBP overexpression. Furthermore, CIRBP overexpression prevented the elevation of ROS induced by Aβ _25-35 treatment by decreasing the activities of oxidative biomarker and increasing the activities of key enzymes in antioxidant system. Taken together, our findings suggested that CIRBP exerted protective effects against neuronal amyloid toxicity via antioxidative and antiapoptotic pathways, which may provide a promising candidate for amyloid-based AD prevention or therapy.

ON-Type Retinal Ganglion Cells are Preferentially Affected in STZ-Induced Diabetic Mice

Purpose

We investigate morphologic and physiologic alterations of ganglion cells (GCs) in a streptozocin (STZ)-induced diabetic mouse model.

Methods

Experiments were conducted in flat-mount retinas of mice 3 months after the induction of diabetes. Changes in morphology of four subtypes of GCs (ON-type RGA2 [ON-RGA2], OFF-type RGA2 [OFF-RGA2], ON-type RGC1 [ON-RGC1], and ON-OFF type RGD2 [ON-OFF RGD2]) were characterized in Thy1-YFP transgenic mice. Using whole-cell patch-clamp recording, passive membrane properties and action potential (AP) firing properties were further investigated in transient ON- and OFF-RGA2 cells.

Results

Morphologic parameters were significantly altered in the dendrites branching in the ON sublamina of the inner plexiform layer (IPL) for ON-RGA2 cells and ON-OFF RGD2 cells. Much less significant changes, if any, were seen in those arborizing in the OFF sublamina of the IPL for OFF-RGA2 and ON-OFF RGD2 cells. No detectable changes in morphology were seen in RGC1 cells. Electrophysiologically, increased resting membrane potentials and decreased membrane capacitance were found in transient ON-RGA2 cells, but not in transient OFF-RGA2 cells. Similar alterations in AP firing properties, such as an increase in AP width and reduction in maximum spiking rate, were shared by these two subtypes. Furthermore, in response to depolarizing current injections, both cells generated more APs suggesting an enhanced excitability of these cells in diabetic conditions.

Conclusions

These differential changes in morphology and electrophysiology in subtypes of GCs may be responsible for reduced contrast sensitivity known to occur during the early stage of diabetic retinopathy.

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

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

Harnessing ionic mechanisms to achieve disease modification in neurodegenerative disorders

Progressive neuronal death is the key pathogenic event leading to clinical symptoms in neurodegenerative disorders (NDDs). Neuroprotective treatments are virtually unavailable, partly because of the marked internal heterogeneity of the mechanisms underlying pathology. Targeted neuroprotection would require deep mechanistic knowledge across the entire aetiological spectrum of each NDD and the development of tailored treatments. Although ideal, this strategy appears challenging, as it would require a degree of characterization of both the disease and the patient that is currently unavailable. The alternate strategy is to search for commonalities across molecularly distinct NDD forms and exploit these for the development of drugs with broad-spectrum efficacy. In this view, mounting evidence points to ionic mechanisms (IMs) as targets with potential therapeutic efficacy across distinct NDD subtypes. The scope of this review is to present clinical and preclinical evidence supporting the link between disruption of IMs and neuronal death in specific NDDs and to critically revise past and ongoing attempts of harnessing IMs for the development of neuroprotective treatments.

Dissecting Endoplasmic Reticulum Unfolded Protein Response (UPRER) in Managing Clandestine Modus Operandi of Alzheimer's Disease

Alzheimer's disease (AD), a neurodegenerative disorder, is most common cause of dementia witnessed among aged people. The pathophysiology of AD develops as a consequence of neurofibrillary tangle formation which consists of hyperphosphorylated microtubule associated tau protein and senile plaques of amyloid-β (Aβ) peptide in specific brain regions that result in synaptic loss and neuronal death. The feeble buffering capacity of endoplasmic reticulum (ER) proteostasis in AD is evident through alteration in unfolded protein response (UPR), where UPR markers express invariably in AD patient's brain samples. Aging weakens UPR^ER causing neuropathology and memory loss in AD. This review highlights molecular signatures of UPR^ER and its key molecular alliance that are affected in aging leading to the development of intriguing neuropathologies in AD. We present a summary of recent studies reporting usage of small molecules as inhibitors or activators of UPR^ER sensors/effectors in AD that showcase avenues for therapeutic interventions.

Resveratrol Attenuates Aβ-Induced Early Hippocampal Neuron Excitability Impairment via Recovery of Function of Potassium Channels

Alzheimer's disease (AD) is an age-related neurodegenerative disease. Amyloid-β (Aβ) is not only the morphological hallmark but also the initiator of the pathology process of AD. As a natural compound found in grapes, resveratrol shows a protective effect on the pathophysiology of AD, but the underlying mechanism is not very clear. This study was to investigate whether resveratrol could attenuate Aβ-induced early impairment in hippocampal neuron excitability and the underlying mechanism. The excitability and voltage-gated potassium currents were examined in rat hippocampal CA1 pyramidal neurons by using whole-cell patch-clamp technique. It was found that Aβ_25-35 increased the excitability of neurons. Resveratrol could reverse the Aβ_25-35-induced increase in the frequency of repetitive firing and the spike half-width of action potential (AP). Moreover, resveratrol can attenuate Aβ_25-35-induced decreases in transient potassium channel (I _A ) and delay rectifier potassium channel (I _K(DR)) of neurons. It was also found that resveratrol could decline the increase of protein kinase A (PKA) and inhibit the activation of PI3K/Akt signaling pathway induced by Aβ_25-35. The results suggest that resveratrol alleviates Aβ_25-35-induced dysfunction in hippocampal CA1 pyramidal neurons via recovery of the function of I _A and I _K(DR) by inhibiting the increase of PKA and the activation of PI3K/Akt signaling pathway.

Capturing intracellular Ca2+ dynamics in computational models of neurodegenerative diseases

Many signaling pathways crucial for homeostatic regulation, synaptic plasticity, apoptosis and immune response depend on Ca²⁺. Ca²⁺ dysregulation disrupts normal function of neurons and neuronal networks. This causes severe motor and cognitive disabilities. Understanding how Ca²⁺ dysregulation triggers disease onset and progression, and affects downstream processes, can help identify targets for treatments. Because of intermingling of molecular pathways, dissecting the role of individual mechanisms and establishing causality is very challenging. Computational models provide a way to decipher these processes. I review some computational models with Ca²⁺ dynamics to illustrate their predictive power, and note where extending those models to capture multiscale interaction of Ca²⁺ dependent molecular pathways can be useful for therapeutic and drug discovery purposes.

Improvement of spatial learning by facilitating large-conductance calcium-activated potassium channel with transcranial magnetic stimulation in Alzheimer's disease model mice

Transcranial magnetic stimulation (TMS) is fragmentarily reported to be beneficial to Alzheimer's patients. Its underlying mechanism was investigated. TMS was applied at 1, 10 or 15 Hz daily for 4 weeks to young Alzheimer's disease model mice (3xTg), in which intracellular soluble amyloid-β is notably accumulated. Hippocampal long-term potentiation (LTP) was tested after behavior. TMS ameliorated spatial learning deficits and enhanced LTP in the same frequency-dependent manner. Activity of the large conductance calcium-activated potassium (Big-K; BK) channels was suppressed in 3xTg mice and recovered by TMS frequency-dependently. These suppression and recovery were accompanied by increase and decrease in cortical excitability, respectively. TMS frequency-dependently enhanced the expression of the activity-dependently expressed scaffold protein Homer1a, which turned out to enhance BK channel activity. Isopimaric acid, an activator of the BK channel, magnified LTP. Amyloid-β lowering was detected after TMS in 3xTg mice. In 3xTg mice with Homer1a knocked out, amyloid-β lowering was not detected, though the TMS effects on BK channel and LTP remained. We concluded that TMS facilitates BK channels both Homer1a-dependently and -independently, thereby enhancing hippocampal LTP and decreasing cortical excitability. Reduced excitability contributed to amyloid-β lowering. A cascade of these correlated processes, triggered by TMS, was likely to improve learning in 3xTg mice.

Tau facilitates Aβ-induced loss of mitochondrial membrane potential independent of cytosolic calcium fluxes in mouse cortical neurons

Alzheimer's disease (AD) is defined by presence of two pathological hallmarks, the intraneuronal neurofibrillary tangle (NFT) formed by abnormally processed tau, and the extracellular amyloid plaques formed primarily by the amyloid beta peptide (Aβ). In AD it is likely that these two proteins act in concert to impair neuronal function, and there is evidence to suggest that one of the key targets on which they converge is the mitochondria. For example, overexpression of a pathologic form of tau in rat primary cortical neurons exacerbates Aβ-induced mitochondrial membrane potential (ΔΨm) loss due to impairment of the calcium (Ca(2+)) buffering capability of mitochondria. However the role of physiological levels of tau in mediating Aβ-induced mitochondrial dysfunction was not examined. Therefore in this present study we used primary neurons from wild type (WT) and tau knockout (tau(-/-)) mice to investigate whether endogenous tau facilitates Aβ-induced ΔΨm loss and alterations in cytosolic calcium (Ca(2+)cyt). Knocking out tau significantly protected mouse primary cortical neurons from loss of ΔΨm caused by low concentrations of Aβ42, which supports our previous findings. However, the absence of tau resulted in significantly greater increases in Ca(2+)cyt in response to Aβ treatment when compared to those observed in WT mouse primary cortical neurons. This unexpected outcome may be explained by findings that suggest tau(-/-) neurons display certain phenotypic abnormalities associated with alterations in Ca(2+)cyt. Overall, data indicate that tau facilitates Aβ-induced mitochondrial dysfunction and this effect is independent of Aβ-induced alterations in Ca(2+)cyt.(1).

Linking aβ42-induced hyperexcitability to neurodegeneration, learning and motor deficits, and a shorter lifespan in an Alzheimer's model

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly. β-amyloid (Aβ) accumulation in the brain is thought to be a primary event leading to eventual cognitive and motor dysfunction in AD. Aβ has been shown to promote neuronal hyperactivity, which is consistent with enhanced seizure activity in mouse models and AD patients. Little, however, is known about whether, and how, increased excitability contributes to downstream pathologies of AD. Here, we show that overexpression of human Aβ42 in a Drosophila model indeed induces increased neuronal activity. We found that the underlying mechanism involves the selective degradation of the A-type K+ channel, Kv4. An age-dependent loss of Kv4 leads to an increased probability of AP firing. Interestingly, we find that loss of Kv4 alone results in learning and locomotion defects, as well as a shortened lifespan. To test whether the Aβ42-induced increase in neuronal excitability contributes to, or exacerbates, downstream pathologies, we transgenically over-expressed Kv4 to near wild-type levels in Aβ42-expressing animals. We show that restoration of Kv4 attenuated age-dependent learning and locomotor deficits, slowed the onset of neurodegeneration, and partially rescued premature death seen in Aβ42-expressing animals. We conclude that Aβ42-induced hyperactivity plays a critical role in the age-dependent cognitive and motor decline of this Aβ42-Drosophila model, and possibly in AD.

Alzheimer's disease-associated peptide Aβ42 mobilizes ER Ca(2+) via InsP3R-dependent and -independent mechanisms

Dysregulation of Ca²⁺ homeostasis is considered to contribute to the toxic action of the Alzheimer's disease (AD)-associated amyloid-β-peptide (Aβ). Ca²⁺ fluxes across the plasma membrane and release from intracellular stores have both been reported to underlie the Ca²⁺ fluxes induced by Aβ₄₂. Here, we investigated the contribution of Ca²⁺ release from the endoplasmic reticulum (ER) to the effects of Aβ₄₂ upon Ca²⁺ homeostasis and the mechanism by which Aβ₄₂ elicited these effects. Consistent with previous reports, application of soluble oligomeric forms of Aβ₄₂ induced an elevation in intracellular Ca²⁺. The Aβ₄₂-stimulated Ca²⁺ signals persisted in the absence of extracellular Ca²⁺ indicating a significant contribution of Ca²⁺ release from the ER Ca²⁺ store to the generation of these signals. Moreover, inositol 1,4,5-trisphosphate (InsP₃) signaling contributed to Aβ₄₂-stimulated Ca²⁺ release. The Ca²⁺ mobilizing effect of Aβ₄₂ was also observed when applied to permeabilized cells deficient in InsP₃ receptors, revealing an additional direct effect of Aβ₄₂ upon the ER, and a mechanism for induction of toxicity by intracellular Aβ₄₂.

Automated parallel recordings of topologically identified single ion channels

Although ion channels are attractive targets for drug discovery, the systematic screening of ion channel-targeted drugs remains challenging. To facilitate automated single ion-channel recordings for the analysis of drug interactions with the intra- and extracellular domain, we have developed a parallel recording methodology using artificial cell membranes. The use of stable lipid bilayer formation in droplet chamber arrays facilitated automated, parallel, single-channel recording from reconstituted native and mutated ion channels. Using this system, several types of ion channels, including mutated forms, were characterised by determining the protein orientation. In addition, we provide evidence that both intra- and extracellular amyloid-beta fragments directly inhibit the channel open probability of the hBK channel. This automated methodology provides a high-throughput drug screening system for the targeting of ion channels and a data-intensive analysis technique for studying ion channel gating mechanisms.

Progressive effect of beta amyloid peptides accumulation on CA1 pyramidal neurons: a model study suggesting possible treatments

Several independent studies show that accumulation of β-amyloid (Aβ) peptides, one of the characteristic hallmark of Alzheimer's Disease (AD), can affect normal neuronal activity in different ways. However, in spite of intense experimental work to explain the possible underlying mechanisms of action, a comprehensive and congruent understanding is still lacking. Part of the problem might be the opposite ways in which Aβ have been experimentally found to affect the normal activity of a neuron; for example, making a neuron more excitable (by reducing the A- or DR-type K⁺ currents) or less excitable (by reducing synaptic transmission and Na⁺ current). The overall picture is therefore confusing, since the interplay of many mechanisms makes it difficult to link individual experimental findings with the more general problem of understanding the progression of the disease. This is an important issue, especially for the development of new drugs trying to ameliorate the effects of the disease. We addressed these paradoxes through computational models. We first modeled the different stages of AD by progressively modifying the intrinsic membrane and synaptic properties of a realistic model neuron, while accounting for multiple and different experimental findings and by evaluating the contribution of each mechanism to the overall modulation of the cell's excitability. We then tested a number of manipulations of channel and synaptic activation properties that could compensate for the effects of Aβ. The model predicts possible therapeutic treatments in terms of pharmacological manipulations of channels' kinetic and activation properties. The results also suggest how and which mechanisms can be targeted by a drug to restore the original firing conditions.

Effect of synthetic aβ peptide oligomers and fluorinated solvents on Kv1.3 channel properties and membrane conductance

The impact of synthetic amyloid β (1–42) (Aβ_1–42) oligomers on biophysical properties of voltage-gated potassium channels Kv 1.3 and lipid bilayer membranes (BLMs) was quantified for protocols using hexafluoroisopropanol (HFIP) or sodium hydroxide (NaOH) as solvents prior to initiating the oligomer formation. Regardless of the solvent used Aβ_1–42 samples contained oligomers that reacted with the conformation-specific antibodies A11 and OC and had similar size distributions as determined by dynamic light scattering. Patch-clamp recordings of the potassium currents showed that synthetic Aβ_1–42 oligomers accelerate the activation and inactivation kinetics of Kv 1.3 current with no significant effect on current amplitude. In contrast to oligomeric samples, freshly prepared, presumably monomeric, Aβ_1–42 solutions had no effect on Kv 1.3 channel properties. Aβ_1–42 oligomers had no effect on the steady-state current (at −80 mV) recorded from Kv 1.3-expressing cells but increased the conductance of artificial BLMs in a dose-dependent fashion. Formation of amyloid channels, however, was not observed due to conditions of the experiments. To exclude the effects of HFIP (used to dissolve lyophilized Aβ_1–42 peptide), and trifluoroacetic acid (TFA) (used during Aβ_1–42 synthesis), we determined concentrations of these fluorinated compounds in the stock Aβ_1–42 solutions by ¹⁹F NMR. After extensive evaporation, the concentration of HFIP in the 100× stock Aβ_1–42 solutions was ∼1.7 μM. The concentration of residual TFA in the 70× stock Aβ_1–42 solutions was ∼20 μM. Even at the stock concentrations neither HFIP nor TFA alone had any effect on potassium currents or BLMs. The Aβ_1–42 oligomers prepared with HFIP as solvent, however, were more potent in the electrophysiological tests, suggesting that fluorinated compounds, such as HFIP or structurally-related inhalational anesthetics, may affect Aβ_1–42 aggregation and potentially enhance ability of oligomers to modulate voltage-gated ion channels and biological membrane properties.

Impact of amyloid-β peptide (1-42) on voltage-gated ion currents in molluscan neurons

Different types of voltage-gated ion currents were recorded in isolated neurons of snail Helix pomatia using the two-microelectrode voltage-clamp technique. Application of amyloid-β peptide (1-42, 1-10 μM) in the bathing solution did not change delayed rectifier K(+)-current and leakage current, but enhanced inactivation of Ca(2+)-current and blocked Ca(2+)-dependent K(+)-current.

Altered intrinsic neuronal excitability and reduced Na+ currents in a mouse model of Alzheimer's disease

Transgenic mice that overproduce beta-amyloid (Aβ) peptides can exhibit central nervous system network hyperactivity. Patch clamp measurements from CA1 pyramidal cells of PSAPP and wild type mice were employed to investigate if altered intrinsic excitability could contribute to such network hyperfunction. At approximately 10 months, when PSAPP mice have a substantial central nervous system Aβ load, resting potential and input resistance were genotype-independent. However, PSAPP mice exhibited a substantially more prominent action potential (AP) burst close to the onset of weak depolarizing current stimuli. The spike afterdepolarization (ADP) was also larger in PSAPP mice. The rate of rise, width and height of APs were reduced in PSAPP animals; AP threshold was unaltered. Voltage-clamp recordings from nucleated macropatches revealed that somatic Na(+) current density was depressed by approximately 50% in PSAPP mice. K(+) current density was unaltered. All genotype-related differences were absent in PSAPP mice aged 5-7 weeks which lack a substantial Aβ load. We conclude that intrinsic neuronal hyperexcitability and changes to AP waveforms may contribute to neurophysiological deficits that arise as a consequence of Aβ accumulation.

Computational study of hippocampal-septal theta rhythm changes due to β-amyloid-altered ionic channels

Electroencephagraphy (EEG) of many dementia patients has been characterized by an increase in low frequency field potential oscillations. One of the characteristics of early stage Alzheimer's disease (AD) is an increase in theta band power (4-7 Hz). However, the mechanism(s) underlying the changes in theta oscillations are still unclear. To address this issue, we investigate the theta band power changes associated with β-Amyloid (Aβ) peptide (one of the main markers of AD) using a computational model, and by mediating the toxicity of hippocampal pyramidal neurons. We use an established biophysical hippocampal CA1-medial septum network model to evaluate four ionic channels in pyramidal neurons, which were demonstrated to be affected by Aβ. They are the L-type Ca²⁺ channel, delayed rectifying K⁺ channel, A-type fast-inactivating K⁺ channel and large-conductance Ca²⁺-activated K⁺ channel. Our simulation results demonstrate that only the Aβ inhibited A-type fast-inactivating K⁺ channel can induce an increase in hippocampo-septal theta band power, while the other channels do not affect theta rhythm. We further deduce that this increased theta band power is due to enhanced synchrony of the pyramidal neurons. Our research may elucidate potential biomarkers and therapeutics for AD. Further investigation will be helpful for better understanding of AD-induced theta rhythm abnormalities and associated cognitive deficits.

Changes in the physiology of CA1 hippocampal pyramidal neurons in preplaque CRND8 mice

Amyloid-β protein (Aβ) is thought to play a central pathogenic role in Alzheimer's disease. Aβ can impair synaptic transmission, but little is known about the effects of Aβ on intrinsic cellular properties. Here we compared the cellular properties of CA1 hippocampal pyramidal neurons in acute slices from preplaque transgenic (Tg+) CRND8 mice and wild-type (Tg-) littermates. CA1 pyramidal neurons from Tg+ mice had narrower action potentials with faster decays than neurons from Tg- littermates. Action potential-evoked intracellular Ca(2+) transients in the apical dendrite were smaller in Tg+ than in Tg- neurons. Resting calcium concentration was higher in Tg+ than in Tg- neurons. The difference in action potential waveform was eliminated by low concentrations of tetraethylammonium ions and of 4-aminopyridine, implicating a fast delayed-rectifier potassium current. Consistent with this suggestion, there was a small increase in immunoreactivity for Kv3.1b in stratum radiatum in Tg+ mice. These changes in intrinsic properties may affect information flow through the hippocampus and contribute to the behavioral deficits observed in mouse models and patients with early-stage Alzheimer's disease.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

Noradrenaline activation of neurotrophic pathways protects against neuronal amyloid toxicity

Degeneration of locus coeruleus (LC) noradrenergic forebrain projection neurons is an early feature of Alzheimer's disease. The physiological consequences of this phenomenon are unclear, but observations correlating LC neuron loss with increased Alzheimer's disease pathology in LC projection sites suggest that noradrenaline (NA) is neuroprotective. To investigate this hypothesis, we determined that NA protected both hNT human neuronal cultures and rat primary hippocampal neurons from amyloid-beta (Abeta(1-42) and Abeta(25-35)) toxicity. The noradrenergic co-transmitter galanin was also effective at preventing Abeta-induced cell death. NA inhibited Abeta(25-35)-mediated increases in intracellular reactive oxygen species, mitochondrial membrane depolarization, and caspase activation in hNT neurons. NA exerted its neuroprotective effects in these cells by stimulating canonical beta(1) and beta(2) adrenergic receptor signaling pathways involving the activation of cAMP response element binding protein and the induction of endogenous nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Treatment with functional blocking antibodies for either NGF or BDNF blocked NA's protective actions against Abeta(1-42) and Abeta(25-35) toxicity in primary hippocampal and hNT neurons, respectively. Taken together, these data suggest that the neuroprotective effects of noradrenergic LC afferents result from stimulating neurotrophic NGF and BDNF autocrine or paracrine loops via beta adrenoceptor activation of the cAMP response element binding protein pathway.

Calcium dysregulation in Alzheimer's disease: from mechanisms to therapeutic opportunities

Calcium is involved in many facets of neuronal physiology, including activity, growth and differentiation, synaptic plasticity, and learning and memory, as well as pathophysiology, including necrosis, apoptosis, and degeneration. Though disturbances in calcium homeostasis in cells from Alzheimer's disease (AD) patients have been observed for many years, much more attention was focused on amyloid-beta (Abeta) and tau as key causative factors for the disease. Nevertheless, increasing lines of evidence have recently reported that calcium dysregulation plays a central role in AD pathogenesis. Systemic calcium changes accompany almost the whole brain pathology process that is observed in AD, including synaptic dysfunction, mitochondrial dysfunction, presenilins mutation, Abeta production and Tau phosphorylation. Given the early and ubiquitous involvement of calcium dysregulation in AD pathogenesis, it logically presents a variety of potential therapeutic targets for AD prevention and treatment, such as calcium channels in the plasma membrane, calcium channels in the endoplasmic reticulum membrane, Abeta-formed calcium channels, calcium-related proteins. The review aims to provide an overview of the current understanding of the molecular mechanisms involved in calcium dysregulation in AD, and an insight on how to exploit calcium regulation as therapeutic opportunities in AD.

Intracellular amyloid induces impairments on electrophysiological properties of cultured human neurons

The role of intracellular amyloid beta (iAbeta) in Alzheimer's disease (AD) initiation and progression attracts more and more attention in recent years. To address whether iAbeta induces early alterations of electrophysiological properties in cultured human primary neurons, we delivered iAbeta with adeno-virus and measured the electrophysiological properties of infected neurons with whole-cell recordings. Our results show that iAbeta induces an increase in neuronal resting membrane potentials, a decrease in K(+) currents and a hyperpolarizing shift in voltage-dependent activation of K(+) currents. These results suggest the electrophysiological impairments induced by iAbeta may be responsible for its neuronal toxicity.

Dysregulation of calcium homeostasis in Alzheimer's disease

The accumulation of oligomeric species of beta-amyloid protein in the brain is considered to be a key factor that causes Alzheimer's disease (AD). However, despite many years of research, the mechanism of neurotoxicity in AD remains obscure. Recent evidence strongly supports the theory that Ca2+ dysregulation is involved in AD. Amyloid proteins have been found to induce Ca2+ influx into neurons, and studies on transgenic mice suggest that this Ca2+ influx may alter neuronal excitability. The identification of a risk factor gene for AD that may be involved in the regulation of Ca2+ homeostasis and recent findings which suggest that presenilins may be involved in the regulation of intracellular Ca2+ stores provide converging lines of evidence that support the idea that Ca2+ dysregulation is a key step in the pathogenesis of AD.

Exploring the mechanism of beta-amyloid toxicity attenuation by multivalent sialic acid polymers through the use of mathematical models

beta-Amyloid peptide (A beta), the primary protein component in senile plaques associated with Alzheimer's disease (AD), has been implicated in neurotoxicity associated with AD. Previous studies have shown that the A beta-neuronal membrane interaction plays a role in the mechanism of A beta toxicity. More specifically, it is thought that A beta interacts with ganglioside rich and sialic acid rich regions of cell surfaces. In light of such evidence, we have used a number of different sialic acid compounds of different valency or number of sialic acid moieties per molecule to attenuate A beta toxicity in a cell culture model. In this work, we proposed various mathematical models of A beta interaction with both the cell membrane and with the multivalent sialic acid compounds, designed to act as membrane mimics. These models allow us to explore the mechanism of action of this class of sialic acid membrane mimics in attenuating the toxicity of A beta. The mathematical models, when compared with experimental data, facilitate the discrimination between different modes of action of these materials. Understanding the mechanism of action of A beta toxicity inhibitors should provide insight into the design of the next generation of molecules that could be used to prevent A beta toxicity associated with AD.

Cortical neurons transgenic for human Abeta40 or Abeta42 have similar vulnerability to apoptosis despite their different amyloidogenic properties

Alzheimer's disease (AD) is a leading cause of chronic dementia in the United States. Its incidence is increasing with an attendant increase in associated health care costs. Amyloid beta peptide (Abeta; a 39-42 amino acid molecule) is the major component of senile plaques, the hallmark lesion of AD. The toxic mechanism of Abeta peptides has not been well characterized. Specifically, the impact of Abeta1-40 (Abeta40) and its slightly longer counterpart fragment, Abeta1-42 (Abeta42), is not clearly understood. It has been suggested that, while Abeta40 might play a more physiologically relevant role, Abeta42 is likely the key amyloidogenic fragment leading to amyloid deposition in the form of plaques in AD, a pivotal process in Alzheimer's pathology. This notion was further supported by a recent study employing transgenic mouse models that expressed either Abeta40 or Abeta42 in the absence of human amyloid beta protein precursor (APP) overexpression. It was found that mice expressing Abeta42, but not Abeta40, developed compact amyloid plaques, congophilic amyloid angiopathy, and diffuse Abeta deposits. Since neuronal loss is one of the hallmark features in AD pathology, we hypothesize that cortical neurons from these two strains of transgenic mice for Abeta might show different vulnerability to cell death induced by classical inducers of apoptosis, such as trophic factor withdrawal (TFW). Contrary to our expectations, we found that, while overexpression of either Abeta40 or 42 significantly increased the vulnerability of primary cortical neurons to WFT-induced cell death, there was no significant difference between the two transgenic lines. Mitochondrial dysfunction, levels of oxidative stress, caspase activation and nuclear fragmentation are increased to about the same extent by both Abeta species in transgenic neurons. We conclude that Abeta40 or Abeta42 induce similar levels of neurotoxicity following TFW in these transgenic mice despite the difference in their amyloidogenic properties.

Common key-signals in learning and neurodegeneration: focus on excito-amino acids, beta-amyloid peptides and alpha-synuclein

In this paper a hypothesis that some special signals ("key-signals" excito-amino acids, beta-amyloid peptides and alpha-synuclein) are not only involved in information handling by the neuronal circuits, but also trigger out substantial structural and/or functional changes in the Central Nervous System (CNS) is introduced. This forces the neuronal circuits to move from one stable state towards a new state, but in doing so these signals became potentially dangerous. Several mechanisms are put in action to protect neurons and glial cells from these potentially harmful signals. However, in agreement with the Red Queen Theory of Ageing (Agnati et al. in Acta Physiol Scand 145:301-309, 1992), it is proposed that during ageing these neuroprotective processes become less effective while, in the meantime, a shortage of brain plasticity occurs together with an increased need of plasticity for repairing the wear and tear of the CNS. The paper presents findings supporting the concept that such key-signals in instances such as ageing may favour neurodegenerative processes in an attempt of maximizing neuronal plasticity.

Integrating multi-unit electrophysiology and plastic culture dishes for network neuroscience

The electrophysiological characterisation of cultured neurons is of paramount importance for drug discovery, safety pharmacology and basic research in the neurosciences. Technologies offering low cost, low technical complexity and potential for scalability towards high-throughput electrophysiology on in vitro neurons would be advantageous, in particular for screening purposes. Here we describe a plastic culture substrate supporting low-complexity multi-unit loose-patch recording and stimulation of developing networks while retaining manufacturability compatible with low-cost and large-scale production. Our hybrid polydimethylsilane (PDMS)-on-polystyrene structures include chambers (6 mm in diameter) and microchannels (25 microm x 3.7 microm x 1 mm) serving as substrate-embedded recording pipettes. Somas are plated and retained in the chambers due to geometrical constraints and their processes grow along the microchannels, effectively establishing a loose-patch configuration without human intervention. We demonstrate that off-the-shelf voltage-clamp, current-clamp and extracellular amplifiers can be used to record and stimulate multi-unit activity with the aid of our dishes. Spikes up to 50 pA in voltage-clamp and 300 microV in current-clamp modes are recorded in sparse and bursting activity patterns characteristic of 1 week-old hippocampal cultures. Moreover, spike sorting employing principal component analysis (PCA) confirms that single microchannels support the recording of multiple neurons. Overall, this work suggests a strategy to endow conventional culture plasticware with added functionality to enable cost-efficient network electrophysiology.

Donepezil is a strong antagonist of voltage-gated calcium and potassium channels in molluscan neurons

Donepezil is an acetylcholinesterase inhibitor used in Alzheimer's disease therapy. The neuroprotective effect of donepezil has been demonstrated in a number of different models of neurodegeneration including beta-amyloid toxicity. Since the mechanisms of neurodegeneration involve the activation of both Ca(2+)- and K(+)-channels, the study of donepezil action on voltage-gated ionic currents looked advisable. In the present study, the action of donepezil on voltage-gated Ca(2+)- and K(+)-channels was investigated on isolated neurons of the edible snail (Helix pomatia) using the two-microelectrodes voltage-clamp technique. Donepezil rapidly and reversibly inhibited voltage activated Ca(2+)-current (I(Ca)) (IC(50)=7.9 microM) and three types of high threshold K(+)-current: Ca(2+)-dependent K(+)-current (I(C)) (IC(50)=6.4 microM), delayed rectifier K(+)-current (I(DR)) (IC(50)=8.0 microM) and fast transient K(+)-current (I(Adepol)) (IC(50)=9.1 microM). The drug caused a dual effect on low-threshold fast transient K(+)-current (I(A)), potentiating it at low (5 microM) concentration, but inhibiting at higher (7 microM and above) concentration. Donepezil also caused a significant hyperpolarizing shift of the voltage-current relationship of I(Ca) (but not of any type of K(+)-current). Results suggest the possible contribution of the blocking effect of donepezil on the voltage-gated Ca(2+)- and K(+)-channels to the neuroprotective effect of the drug.

[Molecular pharmacological studies on potassium channels and their regulatory molecules]

K+ channels play important roles in the control of a large variety of physiological functions such as muscle contraction, neurotransmitter release, hormone secretion, and cell proliferation. Over 100 cloned K+ channel pore-forming alpha and accessory beta subunits have been identified so far. Here, we introduce a series of molecular pharmacological and physiological studies on some types of voltage-dependent K+ channels and Ca2+-activated K+ channels. We examined molecular cloning and functional characterization of novel, fast-inactivating, A-type K+ channel alpha (Kv4.3L) and beta (KChIP2S) subunits predominantly expressed in mammalian heart and found the sites in Kv4 channels for 1) the regulation of voltage dependency and 2) the CaMKII phosphorylation in the C-terminal cytoplasmic domain. Moreover, we found that delayed rectifier-type K+ channels (ERG1 and KCNQ) contribute to the resting membrane conductance in vascular and gastrointestinal smooth muscles. The large-conductance Ca2+-activated K+ (BK) channel is ubiquitously expressed and contributes to diverse physiological processes. Recent reports have shown that a BK-like channel (mitoKCa) is expressed in cardiac mitochondria, suggesting that BK channel openers protect mammalian hearts against ischemic injury. Our studies revealed that BKbeta1 interacts with cytochrome c oxidase I (Cco1) in cardiac mitochondria, and that the activation of BK channels by 17beta-estradiol results in a significant increase in the survival rate of ventricular myocytes. These findings suggest that BKbeta1 may play an important role in the regulation of cell respiration in cardiac myocytes and be a target for the modulation by female gonadal hormones.