Protein inhibitor of activated STAT1 Ser 503 phosphorylation‐mediated Elk‐1 SUMOylation promotes neuronal survival in APP/PS1 mice

Background and Purpose Protein inhibitor of activated STAT1 (PIAS1) is phosphorylated by IKKα at Ser 90 in a PIAS1 E3 ligase activity‐dependent manner. Whether PIAS1 is also phosphorylated at other residues and the functional significance of these additional phosphorylation events are not known. The transcription factor Elk‐1 remains SUMOylated under basal conditions, but the role of Elk‐1 SUMOylation in brain is unknown. Here, we examined the functional significance of PIAS1‐mediated Elk‐1 SUMOylation in Alzheimer's disease (AD) using the APP/PS1 mouse model of AD and amyloid β (Aβ) microinjections in vivo. Experimental Approach Novel phosphorylation site(s) on PIAS1 were identified by LC–MS/MS, and MAPK/ERK‐mediated phosphorylation of Elk‐1 demonstrated using in vitro kinase assays. Elk‐1 SUMOylation by PIAS1 in brain was determined using in vitro SUMOylation assays. Apoptosis in hippocampus was assessed by measuring GADD45α expression by western blotting, and apoptosis of hippocampal neurons in APP/PS1 mice was assessed by TUNEL assay. Key Results Using LC–MS/MS, we identified a novel MAPK/ERK‐mediated phosphorylation site on PIAS1 at Ser 503 and showed this phosphorylation determines PIAS1 E3 ligase activity. In rat brain, Elk‐1 was SUMOylated by PIAS1, which decreased Elk‐1 phosphorylation and down‐regulated GADD45α expression. Moreover, lentiviral‐mediated transduction of Elk‐1‐SUMO1 reduced the number of hippocampal apoptotic neurons in APP/PS1 mice. Conclusions and Implications MAPK/ERK‐mediated phosphorylation of PIAS1 at Ser 503 determines PIAS1 E3 ligase activity. Moreover, PIAS1 mediates SUMOylation of Elk‐1, which functions as an endogenous defence mechanism against Aβ toxicity in vivo. Targeting Elk‐1 SUMOylation could be considered a novel therapeutic strategy against AD.