Objective:The apolipoprotein E (APOE) E4 isoform is the strongest genetic risk factor for sporadic Alzheimer disease (AD). Although APOE is predominantly expressed by astrocytes in the central nervous system, neuronal expression of APOE is of increasing interest in age-related cognitive impairment, neurological injury, and neurodegeneration. Here, we show that endogenous expression of E4 in stem-cell-derived neurons predisposes them to injury and promotes the release of phosphorylated tau. Methods: Induced pluripotent stem cells from 2 unrelated AD patients carrying the E4 allele were corrected to the E3/E3 genotype with the CRISPR/Cas9 system and differentiated into pure cultures of forebrain excitatory neurons without contamination from other cells types. Results: Compared to unedited E4 neurons, E3 neurons were less susceptible to ionomycin-induced cytotoxicity. Biochemically, E4 cells exhibited increased tau phosphorylation and ERK1/2 phosphoactivation. Moreover, E4 neurons released increased amounts of phosphorylated tau extracellularly in an isoform-dependent manner by a heparin sulfate proteoglycan-dependent mechanism. Interpretation: Our results demonstrate that endogenous expression of E4 by stem-cell-derived forebrain excitatory neurons predisposes neurons to calcium dysregulation and ultimately cell death. This change is associated with increased cellular tau phosphorylation and markedly enhanced release of phosphorylated tau. Importantly, these effects are independent of glial APOE. These findings suggest that E4 accelerates spreading of tau pathology and neuron death in part by neuron-specific, glia-independent mechanisms. ANN NEUROL 2019;85:726-739 T he apolipoprotein E (APOE) E4 allele, the strongest genetic risk factor for sporadic Alzheimer disease (AD), differs from the risk-neutral E3 allele by a single nucleotide polymorphism (SNP). 1,2 E4 patients exhibit greater brain atrophy, accumulation of hyperphosphorylated tau protein, and deposition of amyloid, albeit by unclear mechanisms. 3-6 Although most APOE is expressed by astrocytes in the brain, 7-9 neuronal APOE is of increasing interest in age-related cognitive impairment, neurological injury, and neurodegeneration. 10-12 Disease modeling using isogenic stem cells has demonstrated that, compared to E3, expression of E4 leads to distinct transcriptomic differences in multiple neural cell types and increases tau phosphorylation in neurons. 13,14 However, the effects of APOE genotype on neuronal viability and tau release are unknown. Using genome editing and a reductionist human stem cell culture approach, we show that endogenous expression of E4 predisposes pure cultures of forebrain excitatory neurons to injury and promotes release of phosphorylated tau (p-tau). These findings suggest that neuronal APOE can accelerate brain atrophy and the spreading of tau pathology in E4-carrying AD patients independently of glia.
Subjects and Methods
Reprogramming and Culture of Stem CellsFibroblast lines from patients diagnosed with AD (AD1, AG11414; AD...