SORL1 regulates the generation of APOE selectively in human neurons

Abstract

AbstractBackgroundSORL1 is a neuronal sorting receptor that has been strongly implicated in the pathogenesis of Alzheimer’s disease (AD). While SORL1 has structural similarities to low‐density lipoprotein (LDL) receptors, it is unclear if it plays a role in regulating lipoprotein levels in the brain. We aimed to utilize human induced pluripotent stem cells (iPSCs) to understand the intersection of SORL1 and APOE biology and interrogate the potential role of SORL1 as an LDL receptor.MethodWe differentiated iPSC lines derived from the ROS and MAP cohorts that span the cognitive and neuropathological spectrum of aging to neuron and astrocyte fates and performed unbiased proteomic profiling, Western blotting, and ELISA to quantify protein levels of SORL1 and APOE. We next used CRISPR/Cas9 to generate SORL1 null iPSC lines and differentiated to neuron, astrocyte, and microglial fates. In neurons and astrocytes, AD‐relevant phenotypes such as APOE, Abeta, and phospho‐tau were assessed. Rescue experiments were performed in SORL1 KO neurons using pharmacological perturbations and shRNA to elucidate the pathways downstream of loss of SORL1 that lead to altered Abeta, tau and APOE.ResultWe observed a strong correlation between SORL1 and APOE protein expression in neurons derived from ROSMAP cohort individuals that was not apparent in astrocytes. Next, we validated previously reported findings that loss of SORL1 in neurons results in an elevation in Abeta levels. We expanded upon these findings to show that SORL1 null neurons display elevated phospho‐tau levels and a significant reduction of extracellular and intracellular APOE. While enhancing retromer and autophagy function rescued the elevation of phosphorylated tau in SORL1 null neurons, these treatments did not rescue reduced APOE levels. Rather, loss of SORL1 resulted in a reduction in APOE RNA expression, suggesting that loss of SORL1 leads to a downregulation of APOE at the transcriptional level. Integration of profiling data from ROSMAP iNs and SORL1 knock out iNs implicate the transcription factors NFkB and REV‐ERB as potentially mediating the effect of loss of SORL1 on APOE RNA levels.ConclusionSORL1 has a neuron‐specific function in regulating APOE levels that is separable from its role in regulating Abeta and phospho‐tau levels.