Epilepsy and epileptiform activity in late-onset Alzheimer disease: clinical and pathophysiological advances, gaps and conundrums

A growing body of evidence has demonstrated a link between Alzheimer disease (AD) and epilepsy. Late-onset epilepsy and epileptiform activity can precede cognitive deterioration in AD by years, and its presence has been shown to predict a faster disease course. In animal models of AD, amyloid and tau pathology are linked to cortical network hyperexcitability that precedes the first signs of memory decline. Thus, detection of epileptiform activity in AD has substantial clinical importance as a potential novel modifiable risk factor for dementia. In this Review, we summarize the epidemiological evidence for the complex bidirectional relationship between AD and epilepsy, examine the effect of epileptiform activity and seizures on cognition in people with AD, and discuss the precision medicine treatment strategies based on the latest research in human and animal models. Finally, we outline some of the unresolved questions of the field that should be addressed by rigorous research, including whether particular clinicopathological subtypes of AD have a stronger association with epilepsy, and the sequence of events between epileptiform activity and amyloid and tau pathology.

Hidden behavioral fingerprints in epilepsy

Epilepsy is a major disorder affecting millions of people. Although modern electrophysiological and imaging approaches provide high-resolution access to the multi-scale brain circuit malfunctions in epilepsy, our understanding of how behavior changes with epilepsy has remained rudimentary. As a result, screening for new therapies for children and adults with devastating epilepsies still relies on the inherently subjective, semi-quantitative assessment of a handful of pre-selected behavioral signs of epilepsy in animal models. Here, we use machine learning-assisted 3D video analysis to reveal hidden behavioral phenotypes in mice with acquired and genetic epilepsies and track their alterations during post-insult epileptogenesis and in response to anti-epileptic drugs. These results show the persistent reconfiguration of behavioral fingerprints in epilepsy and indicate that they can be employed for rapid, automated anti-epileptic drug testing at scale.

Neuroinflammation: A Common Pathway in Alzheimer's Disease and Epilepsy

BACKGROUND

Neuroinflammation is an innate immunological response of the central nervous system that may be induced by a brain insult and chronic neurodegenerative conditions. Recent research has shown that neuroinflammation may contribute to the initiation of Alzheimer's disease (AD) pathogenesis and associated epileptogenesis.

OBJECTIVE

This systematic review aimed to investigate the available literature on the shared molecular mechanisms of neuroinflammation in AD and epilepsy.

METHODS

The search included in this systematic review was obtained from 5 established databases. A total of 2,760 articles were screened according to inclusion criteria. Articles related to the modulation of the inflammatory biomarkers commonly associated with the progression of AD and epilepsy in all populations were included in this review.

RESULTS

Only 7 articles met these criteria and were chosen for further analysis. Selected studies include both in vitro and in vivo research conducted on rodents. Several neuroinflammatory biomarkers were reported to be involved in the cross-talk between AD and epilepsy.

CONCLUSION

Neuroinflammation was directly associated with the advancement of AD and epilepsy in populations compared to those with either AD or epilepsy. However, more studies focusing on common inflammatory biomarkers are required to develop standardized monitoring guidelines to prevent the manifestation of epilepsy and delay the progression of AD in patients.

Alzheimer's disease and epilepsy: An increasingly recognized comorbidity

Both Alzheimer's disease (AD) and epilepsy are common chronic diseases in older people. Seizures and epileptiform discharges are very prevalent in AD and can occur since any stage of AD. Increasing evidence indicates that AD and epilepsy may be comorbid. Several factors may be related to the underlying mechanism of the comorbidity. Identifying seizures in patients with AD is a challenge because seizures are often clinically non-motor and may overlap with some AD symptoms. Not only seizures but also epileptiform discharges may exacerbate the cognitive decline in AD patients, highlighting the importance of early recognition and treatment. This review provides a comprehensive overview of seizures in AD from multiple aspects to provide more insight.

Bioactive human Alzheimer brain soluble Aβ: pathophysiology and therapeutic opportunities

The accumulation of amyloid-β protein (Aβ) plays an early role in the pathogenesis of Alzheimer's disease (AD). The precise mechanism of how Aβ accumulation leads to synaptic dysfunction and cognitive impairment remains unclear but is likely due to small soluble oligomers of Aβ (oAβ). Most studies have used chemical synthetic or cell-secreted Aβ oligomers to study their pathogenic mechanisms, but the Aβ derived from human AD brain tissue is less well characterized. Here we review updated knowledge on the extraction and characterization of bioactive human AD brain oAβ and the mechanisms by which they cause hippocampal synaptic dysfunction. Human AD brain-derived oAβ can impair hippocampal long-term potentiation (LTP) and enhance long-term depression (LTD). Many studies suggest that oAβ may directly disrupt neuronal NMDA receptors, AMPA receptors and metabotropic glutamate receptors (mGluRs). oAβ also impairs astrocytic synaptic functions, including glutamate uptake, D-serine release, and NMDA receptor function. We also discuss oAβ-induced neuronal hyperexcitation. These results may suggest a multi-target approach for the treatment of AD, including both oAβ neutralization and reversal of glutamate-mediated excitotoxicity.

Astrocyte and glutamate involvement in the pathogenesis of epilepsy in Alzheimer's disease

Alzheimer's disease (AD) can increase the risk of epilepsy by up to 10-fold compared to healthy age-matched controls. However, the pathological mechanisms that underlie this increased risk are poorly understood. Because disruption in brain glutamate homeostasis has been implicated in both AD and epilepsy, this might play a mechanistic role in the pathogenesis of epilepsy in AD. Prior to the formation of amyloid beta (Aβ) plaques, the brain can undergo pathological changes as a result of increased production of amyloid precursor protein (APP) and Aβ oligomers. Impairments in the glutamate uptake ability of astrocytes due to astrogliosis are hypothesized to be an early event occurring before Aβ plaque formation. Astrogliosis may increase the susceptibility to epileptogenesis of the brain via accumulation of extracellular glutamate and resulting excitotoxicity. Here we hypothesize that Aβ oligomers and proinflammatory cytokines can cause astrogliosis and accumulation of extracellular glutamate, which then contribute to the pathogenesis of epilepsy in AD. In this review article, we consider the evidence supporting a potential role of dysfunction of the glutamate-glutamine cycle and the astrocyte in the pathogenesis of epilepsy in AD.

Modulation of Brain Hyperexcitability: Potential New Therapeutic Approaches in Alzheimer's Disease

People with Alzheimer's disease (AD) have significantly higher rates of subclinical and overt epileptiform activity. In animal models, oligomeric Aβ amyloid is able to induce neuronal hyperexcitability even in the early phases of the disease. Such aberrant activity subsequently leads to downstream accumulation of toxic proteins, and ultimately to further neurodegeneration and neuronal silencing mediated by concomitant tau accumulation. Several neurotransmitters participate in the initial hyperexcitable state, with increased synaptic glutamatergic tone and decreased GABAergic inhibition. These changes appear to activate excitotoxic pathways and, ultimately, cause reduced long-term potentiation, increased long-term depression, and increased GABAergic inhibitory remodelling at the network level. Brain hyperexcitability has therefore been identified as a potential target for therapeutic interventions aimed at enhancing cognition, and, possibly, disease modification in the longer term. Clinical trials are ongoing to evaluate the potential efficacy in targeting hyperexcitability in AD, with levetiracetam showing some encouraging effects. Newer compounds and techniques, such as gene editing via viral vectors or brain stimulation, also show promise. Diagnostic challenges include identifying best biomarkers for measuring sub-clinical epileptiform discharges. Determining the timing of any intervention is critical and future trials will need to carefully stratify participants with respect to the phase of disease pathology.

Aberrant expression of PAR bZIP transcription factors is associated with epileptogenesis, focus on hepatic leukemia factor

Epilepsy is a widespread neurological disease characterized by abnormal neuronal activity resulting in recurrent seizures. There is mounting evidence that a circadian system disruption, involving clock genes and their downstream transcriptional regulators, is associated with epilepsy. In this study, we characterized the hippocampal expression of clock genes and PAR bZIP transcription factors (TFs) in a mouse model of temporal lobe epilepsy induced by intrahippocampal injection of kainic acid (KA). The expression of PAR bZIP TFs was significantly altered following KA injection as well as in other rodent models of acquired epilepsy. Although the PAR bZIP TFs are regulated by proinflammatory cytokines in peripheral tissues, we discovered that the regulation of their expression is inflammation-independent in hippocampal tissue and rather mediated by clock genes and hyperexcitability. Furthermore, we report that hepatic leukemia factor (Hlf), a member of PAR bZIP TFs family, is invariably downregulated in animal models of acquired epilepsy, regulates neuronal activity in vitro and its overexpression in dentate gyrus neurons in vivo leads to altered expression of genes associated with seizures and epilepsy. Overall, our study provides further evidence of PAR bZIP TFs involvement in epileptogenesis and points to Hlf as the key player.

Optogenetic intervention of seizures improves spatial memory in a mouse model of chronic temporal lobe epilepsy

OBJECTIVE

To determine if closed-loop optogenetic seizure intervention, previously shown to reduce seizure duration in a well-established mouse model chronic temporal lobe epilepsy (TLE), also improves the associated comorbidity of impaired spatial memory.

METHODS

Mice with chronic, spontaneous seizures in the unilateral intrahippocampal kainic acid model of TLE, expressing channelrhodopsin in parvalbumin-expressing interneurons, were implanted with optical fibers and electrodes, and tested for response to closed-loop light intervention of seizures. Animals that responded to closed-loop optogenetic curtailment of seizures were tested in the object location memory test and then given closed-loop optogenetic intervention on all detected seizures for 2 weeks. Following this, they were tested with a second object location memory test, with different objects and contexts than used previously, to assess if seizure suppression can improve deficits in spatial memory.

RESULTS

Animals that received closed-loop optogenetic intervention performed significantly better in the second object location memory test compared to the first test. Epileptic controls with no intervention showed stable frequency and duration of seizures, as well as stable spatial memory deficits, for several months after the precipitating insult.

SIGNIFICANCE

Many currently available treatments for epilepsy target seizures but not the associated comorbidities, therefore there is a need to investigate new potential therapies that may be able to improve both seizure burden and associated comorbidities of epilepsy. In this study, we showed that optogenetic intervention may be able to both shorten seizure duration and improve cognitive outcomes of spatial memory.

Tau Related Pathways as a Connecting Link between Epilepsy and Alzheimer's Disease

Emerging findings point toward an important interconnection between epilepsy and Alzheimer's disease (AD) pathogenesis. Patients with epilepsy (PWE) commonly exhibit cognitive impairment similar to AD patients, who in turn are at a higher risk of developing epilepsy compared to age-matched controls. To date, no disease-modifying treatment strategy is available for either epilepsy or AD, reflecting an immediate need for exploring common molecular targets, which can delineate a possible mechanistic link between epilepsy and AD. This review attempts to disentangle the interconnectivity between epilepsy and AD pathogenesis via the crucial contribution of Tau protein. Tau protein is a microtubule-associated protein (MAP) that has been implicated in the pathophysiology of both epilepsy and AD. Hyperphosphorylation of Tau contributes to the different forms of human epilepsy and inhibition of the same exerted seizure inhibitions and altered disease progression in a range of animal models. Moreover, Tau-protein-mediated therapy has demonstrated promising outcomes in experimental models of AD. In this review, we discuss how Tau-related mechanisms might present a link between the cause of seizures in epilepsy and cognitive disruption in AD. Untangling this interconnection might be instrumental in designing novel therapies that can minimize epileptic seizures and cognitive deficits in patients with epilepsy and AD.

Precuneus-Dominant Degeneration of Parietal Lobe Is at Risk of Epilepsy in Mild Alzheimer's Disease

Introduction: Alzheimer's disease (AD) is the leading cause of cognitive decline. Epilepsy is a frequent comorbid condition of AD. While previous studies analyzed the risk factors of AD-related epileptic seizures, we still lack biomarkers of epilepsy in mild AD cases.

Purpose: The aim of our study was to analyze the correlations between neuropsychology, cortical thickness, and brain volumetric measurements in mild Alzheimer patients with concomitant epileptic seizures.

Materials and methods: We selected mild AD patients from our database to examine them with structural magnetic resonance imaging, 24 h electroencephalography, and detailed neuropsychology. We made the diagnosis of epilepsy based on epileptology data including neurophysiology. We retrospectively analyzed the neuropsychology pattern, clinical and epidemiologic features, cortical thickness, and volumetric values of mild AD patients with and without overt clinical seizures using covariance weighted general linear model.

Results: We found epileptic seizures in 26% of mild AD patients. Patients with seizures performed worse in visuo-spatial scores than patients without (p = 0.003). Patients with seizures had smaller parietal thickness (p = 0.018), being associated to reduced thickness of left (p = 0.007), and right precunei (p = 0.005). The visuo-spatial performance positively and strongly correlated with the thickness of the parietal lobe (r = 0.67; p = 0.002) and with the volume of the precuneus (r = 0.612; p = 0.005).

Conclusion: Epileptic seizures are common even in mild AD. We found that a prominent deficit in visuo-spatial skills is a red flag for epileptic seizures in the initial phase of AD, indicating the early involvement of parietal lobe in the neurodegenerative process. Because our findings suggest that the degeneration of precuneus is a sensitive marker of seizures associated to mild AD, clinicians need to pay special attention to the pattern of atrophy shown by structural MRI. Our results confirm previous data suggesting that epileptic seizures might be associated to a faster progressing type of AD with the early degeneration of posterior cortical areas.

The overlap between epilepsy and Alzheimer's disease and the consequences for treatment

Introduction: Alzheimer's disease may be associated with both clinical and subclinical epileptic seizure activity. Once regarded as an epiphenomenon, epileptiform activity may, in fact, be an integral part of the Alzheimer's phenotype, and may be not only a symptomatic therapeutic target but also a possible mechanism to retard or prevent disease progression. Areas covered: The authors review clinical research articles with a focus on the semiology, epidemiology, and treatment of seizures in Alzheimer's disease, and also look at some experimental animal model studies which have informed clinical thinking on seizure aetiopathogenesis. The evidence base for treatment decisions is sparse. A brief overview of the clinical assessment of Alzheimer's disease patients considering relevant differential diagnoses and diagnostic pitfalls is presented. Expert opinion: Studies of epileptic seizures in Alzheimer's disease have become more frequent over the last 5-10 years. Understanding of seizure semiology, epidemiology, and possible pathogenesis has increased. However, the optimal management of seizures in this context remains unknown, largely due to the paucity of studies sufficient to examine this question. Clearly, such studies will be required, not only to inform clinicians about symptomatic control of seizures in Alzheimer's disease but also to investigate whether this might impact on disease progression.