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

Abstract

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 a-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.

The Role of Parl and HtrA2 in Striatal Neuronal Injury After Transient Global Cerebral Ischemia

“…12 Moisoi et al 30 showed that HtrA2 is necessary for preventing accumulation of unfolded protein and oxidative stress in mitochondria. A recent study by Li et al 31 reported that HtrA2 is responsible for the maintenance of constitutive autophagy, which can eliminate damaged mitochondria. The exact protective mechanisms of HtrA2, especially under ischemic stress, remain unclear and warrant further investigation.…”

Protease Omi cleaving Hax-1 protein contributes to OGD/R-induced mitochondrial damage in neuroblastoma N2a cells and cerebral injury in MCAO mice

“…In addition, although Hax-1 protein levels were decreased in cells exposed to OGD/R ( Figure 1C), transcription of Hax-1 was not altered (Figure 1D), suggesting a post-translational process of Hax-1. As we previously found that Omi cleaves Hax-1 [24] and that Hax-1 is degraded by the ubiquitin-proteasome pathway after apoptotic stimulation [26] , we performed experiments using a cellular OGD/R model to address how Omi regulates Hax-1 after I/R. In N2a cells that were exposed to OGD/R, an increase of Omi and a decrease of Hax-1 were observed, however, Hax-1 protein levels were dramatically increased after Omi was knocked down (Figure 1E), suggesting that Omi possibly processes Hax-1.…”

“…The Hax-1 related constructs were generated as previously described [24] . Briefly, full length Hax-1 cDNA was amplified from a human fetal brain library (Invitrogen) with the primers 5'-GCGAATTCACCATGAGCCTCTTTGATCTCTTC-3' and 5'-ATGTCGACATCCGGGACCGGAACCAACGTCCC-3', and inserted into a pEGFP-N1 (Clontech, Mountain View, CA, USA) vector at the Hind III and Sal I sites.…”

Protease Omi facilitates neurite outgrowth in mouse neuroblastoma N2a cells by cleaving transcription factor E2F1

“…The oligonucleotides targeting Omi sequences (si-Omi) and the negative control (NC) siRNA have been described previously [27] . The oligonucleotides to target sequences of E2F1 were purchased from Ribobio (Guangzhou, China) and their sequences were as follows: si-E2F1 sense 5'-GGAUCUGGAGACUGACCAU-3', antisense 5'-AUGGUCAGUCUCCAGAUCC-3'.…”

“…The Omi-related plasmids have been described previously [27] . Full-length human E2F1 complementary DNA (cDNA) was amplified using PCR from a human fetal brain cDNA library with the primers 5'-GAGGATCCCCATG-GCCTTGGCCGGGGCC-3' and 5'-GAGAATCCCGAAATC-CAGGGGGTGA-3'.…”