EEG alterations in non-demented individuals related to apolipoprotein E genotype and to risk of Alzheimer disease

Age- and genotype-related neurophysiologic reactivity to oxidative stress in healthy adults

The epsilon4 allele of the apolipoprotein E gene (ApoE), as well as aging increase the risk of Alzheimer's and vascular diseases. Electroencephalogram (EEG) reactivity to hyperventilation (HV) depends on hypocapnia-induced cerebral vasoconstriction, which may be impaired in subjects with subclinical cerebrovascular disease. Quantitative EEG at rest and under 3-minute HV was examined in 125 healthy subjects divided into younger (age range 28-50) and older (age range 51-82) cohorts and stratified by ApoE genotype. The younger ApoE-epsilon4 carriers had excessive EEG reactivity to HV characterized by the manifestation of high-voltage delta, theta activity and sharp waves, and larger HV-induced changes in EEG relative powers than in the younger ApoE-epsilon4 noncarriers. EEG reactivity to HV decreased with aging, and in the ApoE-epsilon4 carriers the decrease was more pronounced than in the ApoE-epsilon4 noncarriers. The older ApoE-epsilon4 carriers had smaller HV-induced changes in EEG relative powers than the older ApoE-epsilon4 noncarriers. A marked decline of EEG reactivity to HV in the older ApoE-epsilon4 carriers suggests the possible impact of vascular factors on the pathogenesis of ApoE-induced Alzheimer disease.

Linkage analysis of autopsy-confirmed familial Alzheimer disease supports an Alzheimer disease locus in 8q24

BACKGROUND/AIMS

We have previously reported the results of an extended genome-wide scan of Swedish Alzheimer disease (AD)-affected families; in this paper, we analyzed a subset of these families with autopsy-confirmed AD.

METHODS

We report the fine-mapping, using both microsatellite markers and single-nucleotide polymorphisms (SNPs), in the observed maximum logarithm of the odds (LOD)-2 unit (LOD(max)-2) region under the identified linkage peak, linkage analysis of the fine-mapping data with additionally analyzed pedigrees, and association analysis of SNPs selected from candidate genes in the linked interval. The subset was made on the criterion of at least one autopsy-confirmed AD case per family, resulting in 24 families.

RESULTS

Linkage analysis of a family subset having at least one autopsy-confirmed AD case showed a significant nonparametric single-point LOD score of 4.4 in 8q24. Fine-mapping under the linkage peak with 10 microsatellite markers yielded an increase in the multipoint (mpt) LOD score from 2.1 to 3.0. SNP genotyping was performed on 21 selected candidate transcripts of the LOD(max)-2 region. Both family-based association and linkage analysis were performed on extended material from 30 families, resulting in a suggestive linkage at peak marker rs6577853 (mpt LOD score = 2.4).

CONCLUSION

The 8q24 region has been implicated to be involved in AD etiology.

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.

Seizures and epilepsy in Alzheimer's disease

Many studies have shown that patients with Alzheimer's disease (AD) are at increased risk for developing seizures and epilepsy. However, reported prevalence and incidence of seizures and relationship of seizures to disease measures such as severity, outcome, and progression vary widely between studies. We performed a literature review of the available clinical and epidemiological data on the topic of seizures in patients with AD. We review seizure rates and types, risk factors for seizures, electroencephalogram (EEG) studies, and treatment responses. Finally, we consider limitations and methodological issues. There is considerable variability in the reported prevalence and incidence of seizures in patients with AD-with reported lifetime prevalence rates of 1.5-64%. More recent, prospective, and larger studies in general report lower rates. Some, but not all, studies have noted increased seizure risk with increasing dementia severity or with younger age of AD onset. Generalized convulsive seizures are the most commonly reported type, but often historical information is the only basis used to determine seizure type and the manifestation of seizures may be difficult to distinguish from other behaviors common in demented patients. EEG has infrequently been performed and reported. Data on treatment of seizures in AD are extremely limited. Similarly, the relationship between seizures and cognitive impairment in AD is unclear. We conclude that the literature on seizures and epilepsy in AD, including diagnosis, risk factors, and response to treatment suffers from methodological limitations and gaps.

The concept of FDG-PET endophenotype in Alzheimer's disease

Often viewed as a potential tool for preclinical diagnosis in early asymptomatic stages of Alzheimer's disease (AD), the term "endophenotype" has acquired a recent popularity in the field. In this review, we analyze the construct of endophenotype-originally designed to discover genes, and examine the literature on potential endophenotypes for the late-onset form of AD (LOAD). We focus on the [18F]-fluoro-2-deoxyglucose (FDG) PET technique, which shows a characteristic pattern of hypometabolism in AD-related regions in asymptomatic carriers of the ApoE E4 allele and in children of AD mothers. We discuss the pathophysiological significance and the positive predictive accuracy of an FDG-endophenotype for LOAD in asymptomatic subjects, and discuss several applications of this endophenotype in the identification of both promoting and protective factors. Finally, we suggest that the term "endophenotype" should be reserved to the study of risk factors, and not to the preclinical diagnosis of LOAD.

Network excitability dysfunction in Alzheimer's disease: insights from in vitro and in vivo models

Recent reports have drawn attention to dysfunctions of intrinsic neuronal excitability and network activity in Alzheimer disease (AD). Here we review the possible causes of these basic dysfunctions and implications for AD, based on in vitro and in vivo findings. We then review the current therapeutic approaches particularly linked to the issue of neuronal excitability in AD.

CONCLUSION

AD is a complex, neurodegenerative disorder. Hippocampal synaptic dysfunction is an early feature of the degenerative process that is clearly linked to memory impairment, the first and major symptom of AD. A growing body of evidence points toward a dysfunction of neuronal networks. Intrinsic neuronal excitability, mainly through profound dysregulation of calcium homeostasis, appears to be largely affected. Consequently, neuronal communication is disturbed. Such cellular defects might underlie cognitive manifestations like fluctuations in cognitive impairment and might also explain several observations obtained with EEG, MEG, MRI, or PET studies, leading to the concept of a disconnection syndrome in AD.

Apolipoprotein E ε4-related thickening of the cerebral cortex modulates selective attention

APOE ε4 carriers have thicker cortex in several neocortical areas than ε4 noncarriers (Espeseth T., Westlye L.T., Fjell A.M., Walhovd K.B., Rootwelt H., Reinvang I., 2008. Accelerated age-related cortical thinning in healthy carriers of apolipoprotein E ε4. Neurobiol. Aging 29, 329-340). To investigate potential physiological and cognitive correlates of these anatomical effects structural magnetic resonance imaging (MRI) data were obtained from 20 APOE ε3 homozygotes and 20 ε4 hetero- and homozygotes, and event-related potentials (ERPs) were recorded during a selective attention task (i.e. three-stimulus oddball). Several areas in both hemispheres were thicker in ε4 carriers than in noncarriers. ε4 carriers also had lower amplitudes to distractors (P3a) and lower target detection accuracy than noncarriers. Mean thickness in cortical areas were correlated with P3a amplitudes, which in turn correlated with accuracy. Path analyses showed that APOE-related difference in accuracy was mediated by APOE-related differences in cortical thickness and P3a amplitudes. The results suggest that APOE ε4 modulates the structural integrity of critical nodes in brain attentional networks.

Intracellular calcium signalling in Alzheimer's disease

More than two decades ago, dysregulation of the intracellular Ca²⁺ homeostasis was suggested to underlie the development of Alzheimer’s disease (AD). This hypothesis was tested in numerous in vitro studies, which revealed multiple Ca²⁺ signalling pathways able to contribute to AD pathology. It remained, however, unclear whether these pathways are also activated in vivo, in cells involved in signal processing in the living brain. Here we review recent data analysing intracellular Ca²⁺ signalling in vivo in the context of previous in vitro findings. We particularly focus on the processes taking place in the immediate vicinity of amyloid plaques and on their possible role for AD-mediated brain dysfunction.

Epilepsy and cognitive impairments in Alzheimer disease

Alzheimer disease (AD) is associated with cognitive decline and increased incidence of seizures. Seizure activity in AD has been widely interpreted as a secondary process resulting from advanced stages of neurodegeneration, perhaps in combination with other age-related factors. However, recent findings in animal models of AD have challenged this notion, raising the possibility that aberrant excitatory neuronal activity represents a primary upstream mechanism that may contribute to cognitive deficits in these models. The following observations suggest that such activity may play a similar role in humans with AD: (1) patients with sporadic AD have an increased incidence of seizures that appears to be independent of disease stage and highest in cases with early onset; (2) seizures are part of the natural history of many pedigrees with autosomal dominant early-onset AD, including those with mutations in presenilin-1, presenilin-2, or the amyloid precursor protein, or with duplications of wild-type amyloid precursor protein; (3) inheritance of the major known genetic risk factor for AD, apolipoprotein E4, is associated with subclinical epileptiform activity in carriers without dementia; and (4) some cases of episodic amnestic wandering and disorientation in AD are associated with epileptiform activity and can be prevented with antiepileptic drugs. Here we review recent experimental data demonstrating that high levels of beta-amyloid in the brain can cause epileptiform activity and cognitive deficits in transgenic mouse models of AD. We conclude that beta-amyloid peptides may contribute to cognitive decline in AD by eliciting similar aberrant neuronal activity in humans and discuss potential clinical and therapeutic implications of this hypothesis.

APOE epsilon4 is associated with postictal confusion in patients with medically refractory temporal lobe epilepsy

This study examined the relationship between the APOE epsilon4 allele and postictal confusion in patients with medically intractable temporal lobe epilepsy (TLE). Patients with at least one epsilon4 allele (n=22) were three times more likely to exhibit postictal confusion (68%) than the 63 patients without epsilon4 (43%). These preliminary results demonstrate that APOE epsilon4 is associated with an increased risk of postictal confusion in patients with medically intractable TLE, suggesting possible dysfunction in neuronal recovery mechanisms.