Apolipoprotein E4 promotes the development of Alzheimer’s disease

By Pooja Toshniwal PahariaFeb 23 2023Reviewed by Benedette Cuffari, M.Sc.

In a recent study published in Nature Aging, researchers characterize the effects of neuronal apolipoprotein E4 (APOE4) on Alzheimer’s disease (AD)-associated pathology in tauopathy mice expressing APOE4.

Study: Neuronal APOE4 removal protects against tau-mediated gliosis, neurodegeneration and myelin deficits. Image Credit: Marko Aliaksandr / Shutterstock.com

What is APOE4?

APOE4 is a strong genetic factor that increases the risk of late-onset AD. Stress and tissue injury induce APOE expression within neurons. Previous studies have reported that neuronal APOE4 is critical to AD-associated processes, such as the loss of inhibitory neurons and memory, as well as the phosphorylation of tau proteins.

However, data on the role of neuronal APOE4 in driving full-scale Alzheimer’s disease pathophysiology are limited. Elucidating the precise contribution of neuronal APOE4 in AD pathogenesis could improve the understanding of cellular origin-specific pathways driving APOE4 effects and likely identify novel targets for developing anti-APOE4 therapies.

About the study

In the present study, researchers evaluate the effects of selective neuronal APOE4 elimination on AD pathophysiological mechanisms, including gliosis, neurodegeneration, tau-associated pathology, neurological dysfunction, and a deficit of myelin.

Floxed APOE4-KI (fE) mice were crossbred with PS19 mice that have been utilized as tauopathy murine models, whereas the hybrid PS19-fE/neuron-specific synapsin-1 promoter (Syn1-Cre) or PS19-fE mice were used for the analysis.

To quantify human APOE4 expression in mice, murine hippocampal lysates were analyzed using enzyme-linked immunosorbent assay (ELISA). Further, single-nucleus ribonucleic acid sequencing g (snRNA-seq) was performed.

Ten-month-old mice were immunostained with the p-tau-specific AT8 antibody to determine whether neuronal APOE elimination affects tau pathology. Neurofibrillary tangles were assessed by thioflavine S (Thio-S) staining, and western blot analysis was performed to determine AT8+ p-tau expression in the murine hippocampus after sequential extraction of highly soluble and less-soluble tau proteins.

An adeno-associated virus-2 (AAV2)-coding human P301S mutant tau protein (AAV2-tau-P301S) was injected into hippocampal tissues of 10-month-old fE murine animals, and tau spread from the injected to the non-injected side was analyzed.

Soma-positive-tau-containing neurons on the non-injected side were quantified to assess neuronal tau propagation, whereas neurons on the injected side were immunostained with the human tau HT7 antibody. Posterolateral ventricle and hippocampal volumes were analyzed, and neuronal loss in various hippocampal regions was quantified.

The cells were immunostained for neuronal nuclear protein (NeuN) and cleaved caspase-3 to identify post-mitotic hippocampal neurons undergoing apoptosis. In addition, immunostaining analyses for the myelin basic protein (MBP), oligodendrocyte progenitor cells (OPCs), and anti-Olig2 were performed.

Neuronal network excitability in the hippocampal cornu ammonis-1 (CA-1) site of PS19-fE4 and PS19-fE3 mice was measured. Murine astrogliosis and microgliosis were assessed, and differential gene expression and differential pathway analyses were performed.

Study findings

Selective elimination of APOE4 from neurons led to significant reductions in tau pathology, gliosis, neurodegeneration, neuronal hyperexcitability, and myelin deficits. The removal of neuronal APOE4 significantly reduced neurodegenerative disease-associated subpopulations of neurons, oligodendrocytes, astrocytes, and microglia, whose accumulation correlated with the severity of tau pathology, neurodegeneration, and myelin deficits.

PS19-fE4/Syn1-Cre murine animals exhibited a 20% decrease in APOE4 expression in relation to the PS19-fE4 murine animals, thus indicating that neuronal APOE4 comprises 20-30% of the total APOE4 expression in cortical and hippocampal tissues.

Likewise, PS19-fE3/Syn1-Cre murine animals demonstrated a 25% decrease in APOE4 expression in relation to the PS19-fE3 counterparts. PS19-fE4/Syn1-Cre animals exhibited an 81% reduction in tauopathy than the PS19-fE4 model.

The elimination of neuronal APOE4 led to significant reductions in the neurofibrillary tangle (NFT)-comprising cell counts in hippocampal tissues of PS19-fE4 murine animals as compared to their S19-fE3 counterparts. PS19-fE3 and PS19-fE4/Syn1-Cre murine animals exhibited significantly reduced levels of the less-soluble p-tau protein.

These findings indicate that neuronal APOE4 likely affects tau expression by promoting tau aggregation, propagation, and phosphorylation and that neuronal APOE4 strongly drove tau-associated pathologies.

Immunostaining with an anti-green fluorescent protein (GFP) showed GFP localization to injected side hippocampal neurons. Neuronal APOE4 drove tau pathology by stimulating tau and/or p-tau propagation between the interconnected regions of the brain.

APOE4 in neurons promoted the apoptosis of postmitotic neurons and drove neuronal dysfunction in the context of tauopathy. Conversely, APOE4 elimination from neurons protected against tau-mediated neurodegeneration and mitigated loss of hippocampal volumes and neurons.

Neuronal APOE4 has a key role in depleting hippocampal oligodendrocytes and OPCs and causing myelin deficits. The elimination of neuronal APOE4 decreased APOE4-promoted disease-associated neuronal (nE4-DA) marker gene expression, whereas neuronal APOE4-promoted disease-associated subpopulations of oligodendrocytes, neurons, dendrites, microglia, and astrocytes.

Contrastingly, disease-protective neuronal clusters, oligodendrocytes, astrocytes, and microglia were enriched. The extent of microgliosis and astrogliosis was reduced by APOE4 removal from neurons.

Conclusions

Overall, the study findings showed that neuronal APOE4 promotes the development of AD pathologies. Thus, the elimination of APOE4 from neurons could mitigate the progressive neurodegenerative alterations occurring in the model of APOE4-driven tau pathology at the cell and tissue level.