IKKI- and IKKI2 Regulation of DNA Damage-Induced Cleavage of Huntingtin

Background Proteolysis of huntingtin (Htt) plays a key role in the pathogenesis of Huntington's disease (HD). However, the environmental cues and signaling pathways that regulate Htt proteolysis are poorly understood. One stimulus may be the DNA damage that accumulates in neurons over time, and the subsequent activation of signaling pathways such as those regulated by IIoB kinase (IKK), which can influence neurodegeneration in HD. Methodology/Principal Findings We asked whether DNA damage induces the proteolysis of Htt and if activation of IKK plays a role. We report that treatment of neurons with the DNA damaging agent etoposide or I3-irradiation promotes cleavage of wild type (WT) and mutant Htt, generating N-terminal fragments of 80a90 kDa. This event requires IKKI2 and is suppressed by IKKI-. Elevated levels of IKKI-, or inhibition of IKKI2 expression by a specific small hairpin RNA (shRNA) or its activity by sodium salicylate, prevents Htt proteolysis and increases neuronal resistance to DNA damage. Moreover, IKKI2 phosphorylates the anti-apoptotic protein Bcl-xL, a modification known to reduce Bcl-xL levels, and activates caspases that can cleave Htt. When IKKI2 expression is blocked, etoposide treatment does not decrease Bcl-xL and activation of caspases is diminished. Similar to silencing of IKKI2, increasing the level of Bcl-xL in neurons prevents etoposide-induced caspase activation and Htt proteolysis. Conclusions/Significance These results indicate that DNA damage triggers cleavage of Htt and identify IKKI2 as a prominent regulator. Moreover, IKKI2-dependent reduction of Bcl-xL is important in this process. Thus, inhibition of IKKI2 may promote neuronal survival in HD as well as other DNA damage-induced neurodegenerative disorders.