INF2-mediated actin filament reorganization confers intrinsic resilience to neuronal ischemic injury

Calabrese, Barbara; Jones, Steven; Yamaguchi-Shiraishi, Yoko; Lingelbach, Michael; Manor, Uri; Svitkina, Tatyana M; Higgs, Henry N; Shih, Andy Y; Halpain, Shelley

During early stages of ischemic brain injury, glutamate receptor hyperactivation mediates neuronal death via osmotic cell swelling. Here we show that ischemia and excess NMDA receptor activation – conditions that trigger neuronal swelling -- cause actin filaments to undergo a rapid and extensive reorganization within the somatodendritic compartment. Normally, F-actin is concentrated within dendritic spines, with relatively little F-actin in the dendrite shaft. However, beginning <5 min after incubation of neurons with NMDA, F-actin depolymerizes within dendritic spines and polymerizes into long, stable filament bundles within the dendrite shaft and soma. A similar “actinification” of the somatodendritic compartment occurs after oxygen/glucose deprivation in vitro, and in mouse brain after photothrombotic stroke in vivo. Following transient, sub-lethal NMDA exposure these actin changes spontaneously reverse within 1-2 hours. A combination of Na+, Cl-, water, and Ca2+ entry are all necessary, but not individually sufficient, for induction of actinification. Spine F-actin depolymerization is also required. Actinification is driven by activation of the F-actin polymerization factor inverted formin-2 (INF2). Silencing of INF2 renders neurons more vulnerable to NMDA-induced membrane leakage and cell death, and formin inhibition markedly increases ischemic infarct severity in vivo. These results show that ischemia-induced actin filament reorganization within the dendritic compartment is an intrinsic pro-survival response that protects neurons from death induced by swelling.