Chronic Evoked Seizures in Young Pre-symptomatic APP/PS1 Mice Induce Serotonin Changes and Accelerate Onset of Alzheimer’s Disease-Related Neuropathology

Basic Medical Sciences Life Sciences Medicine Cell Biology Biochemistry
Alzheimer’s Disease Serotonin Glia Amyloid Corneal Kindling Seizures

APP/PS1 Mouse Study Reveals Link Between Chronic Induced Epilepsy and Alzheimer’s Disease

Background

Alzheimer’s Disease (AD) is the most common type of dementia globally, affecting over 55 million people. The typical pathological feature of AD is the deposition of amyloid-β (Aβ) in the brain. Although Aβ deposition and the mechanisms it triggers play an essential role in the diagnosis and progression of AD, substantial evidence suggests that Aβ deposition is not the only cause of AD progression. Consequently, studying the pathological mechanisms at the early stages of AD, before Aβ plaque deposition, is crucial for understanding and treating AD.

5% of AD cases occur before the age of 65 and are considered early-onset AD (EOAD). These cases are often accompanied by temporal lobe epilepsy, and in recent years, epilepsy as an under-researched complication in AD has garnered increasing attention. Unfortunately, how epilepsy directly affects AD progression and symptom severity remains unclear. This study aims to explore how neuron hyperexcitability induced by epilepsy in the early stages of AD impacts disease progression.

Study Origin

The study was conducted by Aaron Del Pozo, Kevin M. Knox, Leanne M. Lehmann, Stephanie Davidson, Seongheon Leo Rho, Suman Jayadev, and Melissa Barker-Haliski, who are affiliated with the Epilepsy Drug Discovery Center and the Department of Neurology at the University of Washington School of Medicine. The paper was published in the journal Progress in Neurobiology, with its online publication date on March 13, 2024.

Research Procedure

Experiment Setup

Subjects and Methods:

  • Subjects:

    • 2-3 month-old APP/PS1 and PSEN2-N141I transgenic mice, along with control group mice.
    • Both male and female mice were included.

  • Methods:

    • Utilizing the corneal kindling model, a well-studied model of epilepsy. Seizures are induced by electric stimulation on the mouse’s cornea.
    • Experimental mice underwent corneal kindling or mock kindling treatment over two weeks.

Steps:

  1. Corneal Kindling:

    • Bilateral pulsed sine-wave electric stimulation (3 seconds, 60 Hz, 1.6-2.0 mA) was administered to 2-month-old APP/PS1 and PSEN2-N141I mice and their control mice for two weeks.
    • The severity of seizures was recorded, and the time taken to reach the kindling criterion (five consecutive Racine stage 5 seizures) was noted.

  2. Survival Rate Assessment:

    • Survival rates during and after reaching the kindling criterion were evaluated.

  3. Sample Collection:

    • Mice were euthanized 24-72 hours after reaching the kindling criterion; brain tissues were rapidly extracted for tissue slicing and protein detection.

  4. Molecular Biology Testing:

    • Western blotting was used to detect levels of proteins related to the 5-HT pathway (such as tryptophan hydroxylase 2, monoamine oxidase A, serotonin transporter) and AD-related proteins (such as PS1, PS2, Aβ) in the hippocampus and prefrontal cortex.

  5. Immunohistochemical Analysis:

    • Immunohistochemical methods evaluated the number and density of astrocyte (GFAP), microglia (IBA-1), neuron (Neun), and Aβ-positive cells and protein expression in the hippocampus and cortex.

Results

  1. Corneal Kindling Effect:

    • Female APP/PS1 mice were more easily kindled than control mice, exhibiting higher seizure burdens and faster attainment of the kindling criterion.
    • Male APP/PS1 mice did not exhibit the same kindling rate or seizure burden differences observed in females.
    • PSEN2-N141I mice (both male and female) did not show significant differences in kindling rates or seizure burden.

  2. Survival Rate:

    • Female APP/PS1 mice showed significantly increased mortality after reaching the kindling criterion (around 75%).
    • PSEN2-N141I mice did not exhibit significant epilepsy-induced mortality rates.

  3. Molecular Biology and Immunohistochemical Results:

    • In the hippocampus of kindled APP/PS1 mice, expression of tryptophan hydroxylase 2 and monoamine oxidase A proteins was significantly reduced, while serotonin transporter expression increased.
    • APP/PS1 mice did not show Aβ plaque deposition but had increased Aβ protein levels.
    • Astrocyte and microglia responses significantly intensified, especially in the CA3 and dentate gyrus regions.
    • PSEN2-N141I mice did not display noticeable effects.

Conclusion and Significance

This study directly demonstrates for the first time that neuron hyperexcitability induced in the early stages of AD can worsen AD pathology and accelerate death by disrupting the 5-HT signaling pathway. The findings also show that chronic seizures and neuron hyperexcitability can lead to increased Aβ protein levels in APP/PS1 mice without plaque deposition and significantly affect glial cell responses.

The study highlights the potential to slow AD by stopping early and persistent neuron hyperexcitability, calling for more research into potential therapeutic interventions. Addressing neuron hyperexcitability earlier may alleviate AD progression and prolong patient life. This provides new insights and directions for future AD research and treatment.

Research Highlights

  1. Identifying the Impact of Early Neuron Hyperexcitability on AD Progression:

    • Provides first-time evidence that neuron hyperexcitability directly affects disease outcomes before AD pathological features (such as Aβ plaques) form.

  2. Discovery of New Therapeutic Targets:

    • Identifies the critical role of the 5-HT pathway in neuron hyperexcitability and the worsening of AD pathology, offering potential targets for novel treatments.

  3. Application of the Corneal Kindling Model:

    • Demonstrates the effectiveness and controllability of the corneal kindling model in studying the relationship between epilepsy and AD.

  4. Sex Differences:

    • Reveals that female APP/PS1 mice are more prone to seizure kindling than males, underlining the importance of considering sex differences in research.

This study not only delves into the mechanisms between epilepsy and AD but also offers new directions and hope for future interventions aimed at mitigating AD progression through the management of neuron hyperexcitability.