Tissue-specific splicing of Omi stress-regulated endoprotease leads to an inactive protease with a modified PDZ motif

HtrA2 regulates α-Synuclein-mediated mitochondrial reactive oxygen species production in the mitochondria of microglia

Aggregation and misfolding of α-Synuclein (α-Syn), a causative agent for Parkinson's disease (PD), and oxidative stress are tightly implicated in the pathogenesis of PD. Although more than 20 genes including HtrA2 have been identified as causative genes for PD, the molecular mechanisms underlying the pathophysiological functions between HtrA2 and α-Syn in the pathogenesis of PD remain unclear. This study shows that HtrA2 serine protease selectively recognizes and interacts with the NAC region of α-Syn. Interestingly, we found that HtrA2 causes proteolysis of α-Syn to prevent mitochondrial accumulation of α-Syn, thereby inhibiting the production of reactive oxygen species (ROS) in the mitochondria. We have further demonstrated that HtrA2 knockdown promotes α-Syn-mediated mitochondrial ROS production, thereby activating microglial cells. This study is the first to demonstrate that the HtrA2/α-Syn cellular partner may play a crucial role in the pathogenesis of PD and provide new insights into the pathological processes and effective therapeutic strategies for PD.

Omi/HtrA2 pro-apoptotic marker differs in various hepatocellular carcinoma cell lines owing to ped/pea-15 expression level

Omi/HtrA2 promotes cell apoptosis in human cancer cells. Early studies showed that primary hepatocellular carcinoma requires Omi/HtrA2 expression for cell apoptosis. Additionally, the Omi/HtrA2 pro-apoptotic marker demonstrated a difference in some cell types. However, how the Omi/HtrA2 pro-apoptotic marker reacts during the process of hepatocellular carcinoma cell apoptosis remains to be determined. Thus, we investigated the role and possible mechanism of Omi/HtrA2 on hepatocellular carcinoma cell apoptosis using various hepatocellular carcinoma cell lines. The results were analyzed using RT‑qPCR and western blot analysis. In the present study, we found that Omi/HtrA2 was overexpressed in hepatocellular carcinoma cell lines and induced hepatocellular carcinoma cell apoptosis. Additiionally, the only manner in which Omi/HtrA2 participated in cell death in PLC cells may be dependent on IAP-binding. Omi/HtrA2‑inducing HepG2 cell apoptosis may mainly depend on its serine protease activity while both IAP-binding and its serine protease activity participated in Hep3B cell apoptosis. This result suggested that Omi/HtrA2 pro-apoptotic marker differs in various hepatocellular carcinoma cell lines. PLC cells were also devoid of the expression of ped/pea-15 as the substrate of Omi/HtrA2 serine protease while ped/pea-15 was overexpressed in HepG2 and Hep3B cells and ped/pea-15 expression was higher in HepG2 cells than that in Hep3B cells. These results showed that Omi/HtrA2 overexpression promotes hepatocellular carcinoma cell apoptosis and the ped/pea-15 expression level causes this difference of the Omi/HtrA2 pro-apoptotic marker in the various hepatocellular carcinoma cell lines.

Inactivation of Omi/HtrA2 protease leads to the deregulation of mitochondrial Mulan E3 ubiquitin ligase and increased mitophagy

Omi/HtrA2 is a nuclear encoded mitochondrial serine protease with dual and opposite functions that depend entirely on its subcellular localization. During apoptosis, Omi/HtrA2 is released into the cytoplasm where it participates in cell death. While confined in the inter-membrane space of the mitochondria, Omi/HtrA2 has a pro-survival function that may involve the regulation of protein quality control (PQC) and mitochondrial homeostasis. Loss of Omi/HtrA2's protease activity causes the neuromuscular disorder of the mnd2 (motor neuron degeneration 2) mutant mice. These mice develop multiple defects including neurodegeneration with parkinsonian features. Loss of Omi/HtrA2 in non-neuronal tissues has also been shown to cause premature aging. The normal function of Omi/HtrA2 in the mitochondria and how its deregulation causes neurodegeneration or premature aging are unknown. Here we report that the mitochondrial Mulan E3 ubiquitin ligase is a specific substrate of Omi/HtrA2. During exposure to H(2)O(2), Omi/HtrA2 degrades Mulan, and this regulation is lost in cells that carry the inactive protease. Furthermore, we show accumulation of Mulan protein in various tissues of mnd2 mice as well as in Omi/HtrA2(-/-) mouse embryonic fibroblasts (MEFs). This causes a significant decrease of mitofusin 2 (Mfn2) protein, and increased mitophagy. Our work describes a new stress-signaling pathway that is initiated in the mitochondria and involves the regulation of Mulan by Omi/HtrA2 protease. Deregulation of this pathway, as it occurs in mnd2 mutant mice, causes mitochondrial dysfunction and mitophagy, and could be responsible for the motor neuron disease and the premature aging phenotype observed in these animals.

Regulation of Abro1/KIAA0157 during myocardial infarction and cell death reveals a novel cardioprotective mechanism for Lys63-specific deubiquitination

Abro1 (also known as KIAA0157) is a scaffold protein that recruits polypeptides to assemble the BRISC (BRCC36-containing isopeptidase complex) deubiquitinating (DUB) enzyme. The four subunits of BRISC enzyme include Abro1, NBA1, BRE, and BRCC36 proteins. The DUB activity of the BRISC enzyme is exclusively directed against Lys63-linked polyubiquitin that does not have a proteolytic role but regulates protein function. In this report, we identified Abro1 as a specific interactor of THAP5, a zinc finger transcription factor that is involved in G2/M control and apoptosis. Abro1 was predominantly expressed in the heart and its protein level was regulated following experimentally induced myocardial ischemia/reperfusion (MI/R) injury. Furthermore, in patients with coronary artery disease (CAD), there was a dramatic increase in Abro1 protein level in the myocardial infarction (MI) area. Increase in Abro1 leads to a significant reduction in Lys63-linked ubiquitination of specific protein targets. Reducing the Abro1 protein level exacerbated cellular damage and cell death of cardiomyocytes due to MI/R injury. Additionally, overexpression of Abro1 in a heterologous system provided significant protection against oxidative stress-induced apoptosis. In conclusion, our results demonstrate that Abro1 protein level substantially increases in myocardial injury and coronary artery disease and this up-regulation is part of a novel cardioprotective mechanism. In addition, our data suggest a potential new link between Lys63-specific ubiquitination, its modulation by the BRISC DUB enzyme, and the development and progression of heart disease.

Evolutionary conservation of mammalian HTRA3 and its developmental regulation in the rat ovary

The high-temperature requirement factor A (HtrA) family of serine proteases is evolutionarily conserved from bacteria to mammals. We have previously identified Htra3 in the mouse and human (HTRA3) and reported its expression in the ovary. In this study, we analyzed the rat Htra3 gene and determined its developmental regulation in the rat ovary. We localized the rat Htra3 gene on chromosome 14q21 and identified two alternatively spliced mRNA variants. The two protein sequences deduced from these mRNAs enabled the prediction of the domain organization of the two protein isoforms. Our comparative analysis has established that the key gene features of Htra3 including its genomic structure, intron-exon junction and alternative splicing are well conserved among the mouse, rat and human. The similarities are even higher at the levels of primary protein sequence and protein domain architecture, suggesting that the functions of Htra3 are highly conserved during evolution from rodents to primates. We demonstrate that Htra3 expression in the rat ovary is developmentally regulated; expression was initiated on day 12 after birth and up-regulated during ovarian maturation with the highest levels found in the mature cycling ovary. In the mature ovary, Htra3 was expressed in granulosa cells, in a follicle-stage specific manner, with the level of expression being dependent on the positioning of the granulosa cells relative to the oocyte in late stage follicles. The luteinizing granulosa cells of the corpus luteum expressed the highest levels of Htra3. Collectively, these results suggest an important role for Htra3 in ovarian development, granulosa cell differentiation and luteinization.

Immunohistochemical analysis of Omi/HtrA2 expression in prostate cancer and benign prostatic hyperplasia

The serine protease Omi/HtrA2 is released from mitochondria into the cytosol after apoptotic stimuli, inducing apoptosis in a caspase-independent manner through its protease activity and in a caspase-dependent manner by neutralizing the inhibition of inhibitor of apoptosis proteins (IAPs) on caspases. Alteration of apoptosis is essential for cancer development, and cancer cell death by radiation and chemotherapy is largely dependent upon apoptosis. Thus, analysis of the expression status of Omi/HtrA2, a regulator of apoptosis, in cancer tissues is needed for an understanding of cancer development. In the current study we analyzed the expression of Omi/HtrA2 in 65 prostate cancer, 40 benign prostatic hyperplasia and 10 normal prostate specimens by immunohistochemistry. Omi/HtrA2 mRNA levels of in vivo prostate cancer and benign prostatic hyperplasia samples were also assayed by semiquantitative reverse transcription-polymerase chain reaction. Immunopositivity (defined as > or =30%) was observed for Omi/HtrA2 in most of the prostate cancers, and the positive rate of Omi/HtrA2 was lower in the well-differentiated group than in the poorly and moderately differentiated groups (p<0.005). By contrast, the cells in the normal prostate and benign prostatic hyperplasia groups showed no or only weak expression of Omi/HtrA2. Meanwhile, the Omi/HtrA2 mRNA level of prostate cancer is much higher than that of benign prostatic hyperplasia (p<0.001). Taken together, these results suggest that prostate cancer cells in vivo may need Omi/HtrA2 expression for apoptosis, and that Omi/HtrA2 expression might be involved in prostate cancer development.

HtrA2 is up-regulated in the rat testis after experimental cryptorchidism

AIM

The aim of the present study was to elucidate the role of high temperature requirement A2 (HtrA2) in germ cell loss in the heat-stressed testis.

METHODS

We examined the expression of HtrA2, caspase-9 activity and proteolytic activity of HtrA2 in the rat testis, and their in vivo responses to experimental cryptorchid treatment.

RESULTS

Northern analysis revealed the expression of HtrA2 mRNA peaked at days 1 and 7 after cryptorchid treatment. While expression of HtrA2 mRNA was seen in the spermatogonium, spermatocytes and some spermatids in normal adult rat testis, experimental cryptorchidism treatment resulted in a marked increase in its signal intensity in spermatocytes and some spermatids, and the layers of spermatogonium and early primary spermatocytes became negative at days 1 and 7 after the treatment. However, the spermatogonium, Sertoli cells and interstitial cells appeared to have strong intensities at days 14, 28 and 56 after the treatment. Western analysis revealed the expression of HtrA2 protein peaked at day 2 coinciding with the increase of positive spermatogonium, the appearance of protein-positive interstitial cells, and day 28 coinciding with the reappearance of protein-positive interstitial cells. Caspase-9 activity peaked at day 2 and HtrA2 proteolytic activity peaked at day 28. Consequently, the first peak of HtrA2 mRNA expression was followed by the peak of caspase-9 activity and the second peak was followed by the peak of proteolytic activity; however, the second peak of mRNA expression had considerable chronological difference from that of the protein.

CONCLUSION

These findings suggest the probabilities that the heat stress results in germ cell death by a caspase-independent manner with the elevation of HtrA2 proteolytic activity, as well as a caspase-dependent manner with the elevation of caspase-9 activity.

Pattern of expression of HtrA1 during mouse development

The human HtrA family of proteases consists of four members: HtrA1, HtrA2, HtrA3, and HtrA4. In humans the four HtrA homologues appear to be involved in several important functions such as cell growth, apoptosis, and inflammatory reactions, and they control cell fate via regulated protein metabolism. In previous studies it was shown that the expression of HtrA1 was ubiquitous in normal adult human tissues. Here we examined the expression of HtrA1 protein and its corresponding mRNA during mouse embryogenesis using Northern blotting hybridization, RT-PCR, and immunohistochemical staining analyses. Our results indicate that HtrA1 is expressed in a variety of tissues in mouse embryos. Furthermore, this expression is regulated in a spatial and temporal manner. Relatively low levels of HtrA1 mRNA are detected in embryos at the beginning of organogenesis (E8), and the levels of expression increase during late organogenesis (E14-E19). Our results show that HtrA1 was expressed during embryonic development in specific areas where signaling by TGFbeta family proteins plays important regulatory roles. The expression of HtrA1, documented both at mRNA and protein levels by RT-PCR and immunohistochemistry in the developing nervous system, is consistent with a possible role of this protein both in dividing and postmitotic neurons, possibly via its documented inhibitory effects on TGFbeta proteins. An exhaustive knowledge of the different cell- and tissue-specific patterns of expression of HtrA1 in normal mouse embryos is essential for a critical evaluation of the exact role played by this protein during development.

The C-terminal Tail of Presenilin Regulates Omi/HtrA2 Protease Activity

Presenilin mutations are responsible for most cases of autosomal dominant inherited forms of early onset Alzheimer disease. Presenilins play an important role in amyloid beta-precursor processing, NOTCH receptor signaling, and apoptosis. However, the molecular mechanisms by which presenilins regulate apoptosis are not fully understood. Here, we report that presenilin-1 (PS1) regulates the proteolytic activity of the serine protease Omi/HtrA2 through direct interaction with its regulatory PDZ domain. We show that a peptide corresponding to the cytoplasmic C-terminal tail of PS1 dramatically increases the proteolytic activity of Omi/HtrA2 toward the inhibitor of apoptosis proteins and beta-casein and induces cell death in an Omi/HtrA2-dependent manner. Consistent with these results, ectopic expression of full-length PS1, but not PS1 lacking the C-terminal PDZ binding motif, potentiated Omi/HtrA2-induced cell death. Our results suggest that the C terminus of PS1 is an activation peptide ligand for the PDZ domain of Omi/HtrA2 and may regulate the protease activity of Omi/HtrA2 after its release from the mitochondria during apoptosis. This mechanism of Omi/HtrA2 activation is similar to the mechanism of activation of the related bacterial DegS protease by the outer-membrane porins.

PDZ domains-glue and guide

PDZ domains are small globular building blocks that are amongst the most abundant protein interaction domains in organisms. Over the past several years an avalanche of data has implicated these modules in the clustering, targeting and routing of associating proteins. An overview is given of the types of interactions displayed by PDZ domains and how this relates to the current knowledge on their spatial structure. Furthermore, the different levels on which PDZ--ligand binding can be regulated and the consequences of PDZ domain-mediated clustering for activity, routing and targeting of interacting proteins will be addressed. Finally, some cell and animal models that illustrate the impact of PDZ domain-containing proteins on (multi-) cellular processes will be discussed.

Random mutagenesis of PDZ(Omi) domain and selection of mutants that specifically bind the Myc proto-oncogene and induce apoptosis

Omi is a mammalian serine protease that is localized in the mitochondria and released to the cytoplasm in response to apoptotic stimuli. Omi induces cell death in a caspase-dependent manner by interacting with the X-chromosome linked inhibitor of apoptosis protein, as well as in a caspase-independent way that relies on its proteolytic activity. Omi is synthesized as a precursor polypeptide and is processed to an active serine protease with a unique PDZ domain. PDZ domains recognize the extreme carboxyl terminus of target proteins. Internal peptides that are able to fold into a beta-finger are also reported to bind some PDZ domains. Using a modified yeast two-hybrid system, PDZ(Omi) mutants were isolated by their ability to bind the carboxyl terminus of human Myc oncoprotein in yeast as well as in mammalian cells. One such PDZ(m) domain (PDZ-M1), when transfected into mammalian cells, was able to bind to endogenous Myc protein and induce cell death. PDZ-M1-induced apoptosis was entirely dependent on the presence of Myc protein and was not observed when c-myc null fibroblasts were used. Our studies indicate that the PDZ domain of Omi can provide a prototype that could easily be exploited to target specifically and inactivate oncogenes by binding to their unique carboxyl terminus.

Immunohistochemical analysis of Omi/HtrA2 expression in stomach cancer

The serine protease Omi/HtrA2 is released from mitochondria into the cytosol after apoptosis stimuli, inducing apoptosis in a caspase-independent manner through its protease activity and in a caspase-dependent manner by neutralizing the inhibition of inhibitor of apoptosis proteins (IAPs) on caspases. Alteration of apoptosis is essential for cancer development, and cancer cell death by radiation and chemotherapy is largely dependent upon apoptosis. Thus, analysis of the expression status of Omi/HtrA2, a regulator of apoptosis, in cancer tissues is needed for an understanding of cancer development. In the current study, we analyzed the expression of Omi/HtrA2 in 60 advanced gastric adenocarcinomas by immunohistochemistry using a tissue microarray approach. Immunopositivity (defined as >/=30%) was observed for Omi/HtrA2 in 43 (72%) of the 60 cancers. By contrast, the surface mucous cells and mucous neck cells in the normal gastric mucosa showed no or weak expression of Omi/HtrA2. Taken together, these results suggest that stomach cancer cells in vivo may need Omi/HtrA2 expression for apoptosis, and that Omi/HtrA2 expression might be involved in stomach cancer development.

Identification and cloning of two isoforms of human high-temperature requirement factor A3 (HtrA3), characterization of its genomic structure and comparison of its tissue distribution with HtrA1 and HtrA2

In the present study, we identified an additional member of the human high-temperature requirement factor A (HtrA) protein family, called pregnancy-related serine protease or HtrA3, which was most highly expressed in the heart and placenta. We cloned the full-length sequences of two forms (long and short) of human HtrA3 mRNA, located the gene on chromosome 4p16.1, determined its genomic structure and revealed how the two mRNA variants are produced through alternative splicing. The alternative splicing was also verified by Northern blotting. Four distinct domains were found for the long form HtrA3 protein: (i) an insulin/insulin-like growth factor binding domain, (ii) a Kazal-type S protease-inhibitor domain, (iii) a trypsin protease domain and (iv) a PDZ domain. The short form is identical to the long form except it lacks the PDZ domain. Comparison of all members of human HtrA proteins, including their isoforms, suggests that both isoforms of HtrA3 represent active serine proteases, that they may have different substrate specificities and that HtrA3 may have similar functions to HtrA1. All three HtrA family members showed very different mRNA-expression patterns in 76 human tissues, indicating a specific function for each. Interestingly, both HtrA1 and HtrA3 are highly expressed in the placenta. Identification of the tissue-specific function of each HtrA family member is clearly of importance.

Characterization of a novel and specific inhibitor for the pro-apoptotic protease Omi/HtrA2

Omi/HtrA2 is a mammalian serine protease with high homology to bacterial HtrA chaperones. Omi/HtrA2 is localized in mitochondria and is released to the cytoplasm in response to apoptotic stimuli. Omi/HtrA2 induces cell death in a caspase-dependent manner by interacting with the inhibitor of apoptosis protein as well as in a caspase-independent manner that relies on its protease activity. We describe the identification and characterization of a novel compound as a specific inhibitor of the proteolytic activity of Omi/HtrA2. This compound (ucf-101) was isolated in a high throughput screening of a combinatorial library using bacterially made Omi-(134-458) protease and fluorescein-casein as a generic substrate. ucf-101 showed specific activity against Omi/HtrA2 and very little activity against various other serine proteases. This compound has a natural fluorescence that was used to monitor its ability to enter mammalian cells. ucf-101, when tested in caspase-9 (-/-) null fibroblasts, was found to inhibit Omi/HtrA2-induced cell death.

CIAP1 and the serine protease HTRA2 are involved in a novel p53-dependent apoptosis pathway in mammals

Recently a Drosophila p53 protein has been identified that mediates apoptosis via a novel pathway involving the activation of the Reaper gene and subsequent inhibition of the inhibitors of apoptosis (IAPs). The present study found that CIAP1, a major mammalian homolog of Drosophila IAPs, is irreversibly inhibited (cleaved) during p53-dependent apoptosis and this cleavage is mediated by a serine protease. Serine protease inhibitors that block CIAP1 cleavage inhibit p53-dependent apoptosis. Furthermore, activation of the p53 protein increases the transcription of the HTRA2 gene, which encodes a serine protease that interacts with CIAP1 and potentiates apoptosis. These results demonstrate that the mammalian p53 protein may activate apoptosis through a novel pathway functionally similar to that in Drosophila, which involves HTRA2 and subsequent inhibition of CIAP1 by cleavage.

The HtrA family of proteases: implications for protein composition and cell fate

Cells precisely monitor the concentration and functionality of each protein for optimal performance. Protein quality control involves molecular chaperones, folding catalysts, and proteases that are often heat shock proteins. One quality control factor is HtrA, one of a new class of oligomeric serine proteases. The defining feature of the HtrA family is the combination of a catalytic domain with at least one C-terminal PDZ domain. Here, we discuss the properties and roles of this ATP-independent protease chaperone system in protein metabolism and cell fate.

A genomic perspective on human proteases

Over 400 human proteases documented in secondary databases can already be delineated in genomic sequence. A Genome Ontology annotation of 30585 sequences in the provisional human proteome set recognises 498 proteases, i.e. 1.6%. Homology searches against finished sequence and comparisons between mouse and zebrafish are likely to increase this total. However, the data already indicate that the mechanistic class, sequence family and domain distribution of the genomic complement of proteases is unlikely to shift significantly from that already observed in the transcript data. Genomically derived novel sequences will require bioinformatic analysis and biochemical verification. The increasing availability of annotated genomic data will enable studies of splice variants, transcriptional control, polymorphisms, pseudogenes, inactive homologues and evolution. Comparative work on complete human protease families should produce a more integrated picture of their biochemistry and physiology. Genomic data will also lead to the identification of new protease involvement in disease processes and their evaluation as drug targets.