Overexpressing low-density lipoprotein receptor reduces tau-associated neurodegeneration via apoE-dependent and independent mechanisms

APOE is the strongest genetic risk factor for late-onset Alzheimer’s disease. ApoE exacerbates tau-associated neurodegeneration by driving microglial activation. However, how apoE regulates microglial activation and whether targeting apoE is therapeutically beneficial in tauopathy is unclear. Here we show that overexpressing a low-density lipoprotein receptor (LDLR) transgene in P301S tau transgenic mice markedly reduces brain apoE and ameliorates tau pathology and neurodegeneration. ApoE specifically interacts with a high-molecular-weight tau species, and highly correlates with phospho-tau and insoluble tau levels. Microglial expression of the LDLR transgene reduces intracellular apoE and is associated with less microglial activation. snRNA-seq analysis of apoE-deficient or LDLR-overexpressing brains reveals that apoE deficiency drives microglial catabolism and increases the oligodendrocyte progenitor cell population. LDLR overexpression shares overlapping mechanisms, but uniquely upregulates microglial expression of specific ion channels and neurotransmitter receptors in tauopathy. A subset of disease-associated astrocytes with both neuroprotective and neurotoxic gene signatures is also identified. 7-8 biologically independent mouse hippocampi were pooled together as a single sample per genotype. Each sample was individually processed and sequenced. A total of 6 groups (genotypes) were analyzed. Droplet-based 3′ end massively parallel single-cell RNA sequencing was performed by isolating single nuclei from flash-frozen mouse hippocampus through sucrose gradient and libraries were prepared using Chromium Single Cell 3′ Reagent Kits according to manufacturer’s protocol (10x Genomics). The generated scRNAseq libraries were sequenced using Illumina sequencers.