Human APOER2 Isoforms Have Differential Cleavage Events and Synaptic Properties

Abstract

Human apolipoprotein E receptor 2 (APOER2) is a type I transmembrane protein with a large extracellular domain (ECD) and a short cytoplasmic tail. APOER2-ECD contains several ligand-binding domains (LBDs) that are organized into exons with aligning phase junctions, which allows for in-frame exon cassette splicing events. We have identified 25 human APOER2 isoforms from cerebral cortex using gene-specific APOER2 primers, where the majority are exon-skipping events within the N-terminal LBD regions compared with six identified in the heart. APOER2 undergoes proteolytic cleavage in response to ligand binding that releases a C-terminal fragment (CTF) and transcriptionally active intracellular domain (ICD). We tested whether the diversity of human brain-specific APOER2 variants affects APOER2 cleavage. We found isoforms with differing numbers of ligand-binding repeats generated different amounts of CTFs compared with full-length APOER2 (APOER2-FL). Specifically, APOER2 isoforms lacking exons 5–8 (Dex5–8) and lacking exons 4–6 (Dex4–6) generated the highest and lowest amounts of CTF generation, respectively, in response to APOE peptide compared with APOER2-FL. The differential CTF generation of Dex5–8 and Dex4–6 coincides with the proteolytic release of the ICD, which mediates transcriptional activation facilitated by the Mint1 adaptor protein. Functionally, we demonstrated loss of mouse Apoer2 decreased miniature event frequency in excitatory synapses, which may be because of a decrease in the total number of synapses and/or VAMP2 positive neurons. Lentiviral infection with human APOER2-FL or Dex4–6 isoform in Apoer2 knockout neurons restored the miniature event frequency but not Dex5–8 isoform. These results suggest that human APOER2 isoforms have differential cleavage events and synaptic properties.