Omi/HtrA2 is a positive regulator of autophagy that facilitates the degradation of mutant proteins involved in neurodegenerative diseases

Omi, also known as high temperature requirement factor A2 (HtrA2), is a serine protease that was originally identified as a proapoptotic protein. Like Smac/Diablo, it antagonizes inhibitor of apoptosis proteins when released into the cytosol on apoptotic stimulation. Loss of its protease activity in mnd2 (motor neuron degeneration 2) mice is associated with neurodegeneration. However, the detailed mechanisms by which Omi regulates the pathogenesis of neurodegenerative disease remain largely unknown. We report here that Omi participates in the pivotal cellular degradation process known as autophagy. It activates autophagy through digestion of Hax-1, a Bcl-2 family-related protein that represses autophagy in a Beclin-1 (mammalian homologue of yeast ATG6)-dependent pathway. Moreover, Omi-induced autophagy facilitates the degradation of neurodegenerative proteins such as pathogenic A53T α-synuclein and truncated polyglutamine-expanded huntingtin, as well as the endogenous autophagy substrate p62. Knockdown of Omi decreases the basal level of autophagy and increases the level of the above target proteins. Furthermore, S276C Omi, the protease-defective mutant found in mnd2 mice, fails to regulate autophagy. Increased autophagy substrates and the formation of aggregate structures are observed in the brains of mnd2 mice. These results identify Omi as a novel regulator of autophagy and suggest that Omi might be important in the cellular quality control of proteins involved in neurodegenerative diseases.