Apolipoprotein E4, inhibitory network dysfunction, and Alzheimer's disease.

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD), increasing risk and decreasing age of disease onset. Many studies have demonstrated the detrimental effects of apoE4 in varying cellular contexts. However, the underlying mechanisms explaining how apoE4 leads to cognitive decline are not fully understood. Recently, the combination of human induced pluripotent stem cell (hiPSC) modeling of neurological diseases in vitro and electrophysiological studies in vivo have begun to unravel the intersection between apoE4, neuronal subtype dysfunction or loss, subsequent network deficits, and eventual cognitive decline. In this review, we provide an overview of the literature describing apoE4's detrimental effects in the central nervous system (CNS), specifically focusing on its contribution to neuronal subtype dysfunction or loss. We focus on γ-aminobutyric acid (GABA)-expressing interneurons in the hippocampus, which are selectively vulnerable to apoE4-mediated neurotoxicity. Additionally, we discuss the importance of the GABAergic inhibitory network to proper cognitive function and how dysfunction of this network manifests in AD. Finally, we examine how apoE4-mediated GABAergic interneuron loss can lead to inhibitory network deficits and how this deficit results in cognitive decline. We propose the following working model: Aging and/or stress induces neuronal expression of apoE. GABAergic interneurons are selectively vulnerable to intracellularly produced apoE4, through a tau dependent mechanism, which leads to their dysfunction and eventual death. In turn, GABAergic interneuron loss causes hyperexcitability and dysregulation of neural networks in the hippocampus and cortex. This dysfunction results in learning, memory, and other cognitive deficits that are the central features of AD

Chimeric Disease Modeling of Apolipoprotein E4 (ApoE4) Toxicity in Human Neurons

Human induced pluripotent stem cells (hiPSCs) provide a unique opportunity to study the human specific aspects of human disorders such as Alzheimer’s disease (AD). However, in vitro systems are inherently artificial and lack the complexity of the in vivo environment. Here we demonstrate how chimeric disease modeling, where we transplanted mixed hiPSC-derived neurons (MNs), comprised primarily of excitatory neurons, and enriched inhibitory neurons (INs) into the mouse hippocampus, allow us to model apoE4 toxicity in different subtypes of human neurons within the in vivo environment. Seven to eight months after transplantation, hiPSC-derived neurons survive and functionally integrate into the mouse brain. However, unlike apoE3 knock-in (apoE3-KI) and apoE4-KI mouse neurons, hiPSC-derived neurons generate A aggregates, with INs generating significantly less than MNs. ApoE4/4 mouse microglia display significant deficits in their ability to phagocytose A aggregates generated by hiPSC-derived neurons, resulting in an increase in the number of aggregates in the apoE4/4 mouse brain. Interestingly, apoE4/4 hiPSC-derived INs are selectively vulnerable to the toxicity of the apoE4/4 brain and display elevated tau-phosphorylation relative to MN transplants. This novel in vivo chimeric model for apoE4/4 toxicity displays human-specific pathological features and allows us to identify how the cellular source of apoE4 relates to pathological response in different neuronal subtypes in the course of Alzheimer’s disease

Chimeric Disease Modeling of Apolipoprotein E4 (ApoE4) Toxicity in Human Neurons

Human induced pluripotent stem cells (hiPSCs) provide a unique opportunity to study the human specific aspects of human disorders such as Alzheimer’s disease (AD). However, in vitro systems are inherently artificial and lack the complexity of the in vivo environment. Here we demonstrate how chimeric disease modeling, where we transplanted mixed hiPSC-derived neurons (MNs), comprised primarily of excitatory neurons, and enriched inhibitory neurons (INs) into the mouse hippocampus, allow us to model apoE4 toxicity in different subtypes of human neurons within the in vivo environment. Seven to eight months after transplantation, hiPSC-derived neurons survive and functionally integrate into the mouse brain. However, unlike apoE3 knock-in (apoE3-KI) and apoE4-KI mouse neurons, hiPSC-derived neurons generate A aggregates, with INs generating significantly less than MNs. ApoE4/4 mouse microglia display significant deficits in their ability to phagocytose A aggregates generated by hiPSC-derived neurons, resulting in an increase in the number of aggregates in the apoE4/4 mouse brain. Interestingly, apoE4/4 hiPSC-derived INs are selectively vulnerable to the toxicity of the apoE4/4 brain and display elevated tau-phosphorylation relative to MN transplants. This novel in vivo chimeric model for apoE4/4 toxicity displays human-specific pathological features and allows us to identify how the cellular source of apoE4 relates to pathological response in different neuronal subtypes in the course of Alzheimer’s disease

In Vivo Chimeric Alzheimer’s Disease Modeling of Apolipoprotein E4 Toxicity in Human Neurons

Despite its clear impact on Alzheimer's disease (AD) risk, apolipoprotein (apo) E4's contributions to AD etiology remain poorly understood. Progress in answering this and other questions in AD research has been limited by an inability to model human-specific phenotypes in an in vivo environment. Here we transplant human induced pluripotent stem cell (hiPSC)-derived neurons carrying normal apoE3 or pathogenic apoE4 into human apoE3 or apoE4 knockin mouse hippocampi, enabling us to disentangle the effects of apoE4 produced in human neurons and in the brain environment. Using single-nucleus RNA sequencing (snRNA-seq), we identify key transcriptional changes specific to human neuron subtypes in response to endogenous or exogenous apoE4. We also find that Aβ from transplanted human neurons forms plaque-like aggregates, with differences in localization and interaction with microglia depending on the transplant and host apoE genotype. These findings highlight the power of in vivo chimeric disease modeling for studying AD

Gain of toxic apolipoprotein E4 effects in human iPSC-derived neurons is ameliorated by a small-molecule structure corrector.

Efforts to develop drugs for Alzheimer's disease (AD) have shown promise in animal studies, only to fail in human trials, suggesting a pressing need to study AD in human model systems. Using human neurons derived from induced pluripotent stem cells that expressed apolipoprotein E4 (ApoE4), a variant of the APOE gene product and the major genetic risk factor for AD, we demonstrated that ApoE4-expressing neurons had higher levels of tau phosphorylation, unrelated to their increased production of amyloid-β (Aβ) peptides, and that they displayed GABAergic neuron degeneration. ApoE4 increased Aβ production in human, but not in mouse, neurons. Converting ApoE4 to ApoE3 by gene editing rescued these phenotypes, indicating the specific effects of ApoE4. Neurons that lacked APOE behaved similarly to those expressing ApoE3, and the introduction of ApoE4 expression recapitulated the pathological phenotypes, suggesting a gain of toxic effects from ApoE4. Treatment of ApoE4-expressing neurons with a small-molecule structure corrector ameliorated the detrimental effects, thus showing that correcting the pathogenic conformation of ApoE4 is a viable therapeutic approach for ApoE4-related AD

Neuronal ApoE upregulates MHC-I expression to drive selective neurodegeneration in Alzheimer’s disease

Selective neurodegeneration is a critical causal factor in Alzheimer's disease (AD); however, the mechanisms that lead some neurons to perish, whereas others remain resilient, are unknown. We sought potential drivers of this selective vulnerability using single-nucleus RNA sequencing and discovered that ApoE expression level is a substantial driver of neuronal variability. Strikingly, neuronal expression of ApoE-which has a robust genetic linkage to AD-correlated strongly, on a cell-by-cell basis, with immune response pathways in neurons in the brains of wild-type mice, human ApoE knock-in mice and humans with or without AD. Elimination or over-expression of neuronal ApoE revealed a causal relationship among ApoE expression, neuronal MHC-I expression, tau pathology and neurodegeneration. Functional reduction of MHC-I ameliorated tau pathology in ApoE4-expressing primary neurons and in mouse hippocampi expressing pathological tau. These findings suggest a mechanism linking neuronal ApoE expression to MHC-I expression and, subsequently, to tau pathology and selective neurodegeneration