Targeting Nell2 and GABAergic Dysfunction to Mitigate APOE4-Driven Pathogenesis in Alzheimer's Disease - PROJECT SUMMARY The complexity and multifactorial nature of Alzheimer’s disease (AD) pose unique challenges for mechanistic studies and developing therapies. Apolipoprotein (apo) E4, the major genetic risk factor for AD, lowers the age of disease onset in a gene dose–dependent manner. In most clinical studies, APOE4 carriers account for 60– 75% of AD cases, highlighting the importance of APOE4 in AD pathogenesis. Longitudinal studies show that APOE4’s detrimental effect on cognition depends on age and occurs before typical signs of AD arise. APOE4 has been linked to subclinical epileptiform activity during stress, and APOE4 carriers have increased hippocampal activity at rest and in response to memory tasks, the extent of which predicts future memory decline. Reducing hippocampal hyperactivity with the antiepileptic drug levetiracetam improved cognition in amnestic mild cognitive impairment (MCI) patients and in AD mouse models, confirming that network hyperactivity promotes AD-related cognitive dysfunction. Challenges in AD research are to fully understand how APOE4, sex, and aging interact in a cell type-specific manner to contribute to AD pathophysiology, and what factors determine individual neuron’s susceptibility or resilience to disease. Our published and preliminary data show that from a young age, female APOE4 knock-in (E4-KI) mice exhibit region-specific hippocampal network hyperexcitability, which predicts learning and memory impairment in old age. This early network pathology is driven by cell type- specific effects of neuronal APOE4 and depends on the growth factor and intracellular signaling protein Nell2, leading to the atrophy and hyperexcitability of vulnerable hippocampal excitatory neurons. Consequently, early network hyperactivity results in the progressive and sex-dependent impairment and death of susceptible APOE4- expressing hilar inhibitory interneurons, ultimately culminating in network failure and cognitive impairment in old age. This proposal aims to (1) investigate the mechanism of action and validate the protective network and behavioral outcomes of Nell2 deletion in hippocampal excitatory neurons of E4-KI mice, and (2) examine the transcriptomic and molecular mechanisms of APOE4-, sex-, and age-dependent dysfunction and selective vulnerability of inhibitory interneurons. We will use a multi-level approach with cutting-edge methods, from single- nucleus RNA sequencing from individual neurons with CRISPR-based target validation, to in-vivo brain activity recording and behavioral testing. The outcomes of the proposed study will provide crucial insights into the mechanisms behind selective APOE4-induced neuronal dysfunction and help identify potential therapeutic targets for mitigating age- and sex-dependent cognitive decline in Alzheimer's disease.
