Early and late events induced by polyQ-expanded proteins: identification of a common pathogenic property of polyq-expanded proteins.

To find a common pathogenetic trait induced by polyQ-ex- panded proteins, we have used a conditional expression system in PC12 cells to tune the expression of these proteins and ana- lyze the early and late consequences of their expression. We find that expression for 3 h of a polyQ-expanded protein stim- ulates cellular reactive oxygen species (ROS) levels and signifi- cantly reduces the mitochondrial electrochemical gradient. 24 –36 h later, ROS induce DNA damage and activation of the checkpoint kinase, ATM. DNA damage signatures are reversi- ble and persist as long as polyQ-expanded proteins are ex- pressed. Transcription of neural and stress response genes is down-regulated in these cells. Selective inhibition of ATM or histone deacetylase rescues transcription and restores the ex- pression of silenced genes. Eventually, after 1 week, the ex- pression of polyQ-expanded protein also induces endoplasmic reticulum stress. As to the primary mechanism responsible for ROS generation, we find that polyQ-expanded proteins, in- cluding native Ataxin-2 and Huntingtin, are selectively seques- tered in the lipid raft membrane compartment and interact with gp91, the membrane NADPH-oxidase subunit. Selective inhibition of NADPH oxidase or silencing of H-Ras signaling dissolves the aggregates and eliminates DNA damage. We sug- gest that targeting of the polyQ-expanded proteins to the lipid rafts activates the resident NADPH oxidase. This triggers a signal linking H-Ras, ROS, and ERK1/2 that maintains and propagates the ROS wave to the nucleus. This mechanism may represent the common pathogenetic signature of all polyQ- expanded proteins independently of the specific context or the function of the native wild type protein.