sSORLA triggers activation of the EGFR/ERK pathway in neurons. A, Phosphoproteomic characterization of SORLA TG versus WT mouse cortex. Lysates from 3-month-old SORLA TG and WT cortex (n = 4 per genotype) were subjected to phosphoproteomic analysis by mass spectrometry. Differentially enriched phosphopeptides are shown in the volcano plot. Dot sizes indicate relative phosphopeptide abundance (MS1 intensity sum across samples; see Materials and Methods). Color schemes represent statistical test SE between samples as indicated. Phosphopeptides detected exclusively in SORLA TG cortex are shown in the adjacent magnified plot. Components of the KEGG MAPK signaling pathway identified by GO DAVID analysis are indicated in the individual plots, where relevant MAPK signaling components are represented in orange (MAPK pathway components) or red (upstream Ras/MAPK regulators). B, Top 15 KEGG pathways identified from upregulated phosphopeptides in SORLA TG hippocampus. C, Components within the KEGG MAPK signaling pathway with upregulated peptides identified in SORLA TG hippocampus. Components of the canonical EGFR/MAPK pathway are highlighted in blue. Mapped mouse and orthologous human phosphoresidues are shown. D, Schematic model depicting putative activation of the MAPK pathway, potentially through SORLA interaction and activation of the EGF receptor. EGFR S991 phosphorylation (mouse S993) was identified through proteomic analysis, and pY residues, such as EGFR pY1173 associated with EGF stimulation and downstream Ras/ERK activation, were identified by immunoblot analysis in response to sSORLA treatment.