Translocation of the Serine Protease Omi/HtrA2 from Mitochondria into the Cytosol Upon Seizure-Induced Hippocampal Injury in the Neonatal Rat Brain

Role of high-temperature requirement serine protease A 2 in rheumatoid inflammation

BACKGROUND

High-temperature requirement serine protease A 2 (HtrA2) is known to be involved in growth, unfolded protein response to stress, apoptosis, and autophagy. However, whether HtrA2 controls inflammation and immune response remains elusive.

METHODS

Expression of HtrA2 in the synovial tissue of patients was examined using immunohistochemistry and immunofluorescence staining. Enzyme-linked immunosorbent assay was used to determine the concentrations of HtrA2, interleukin-6 (IL-6), interleukin-8 (IL-8), chemokine (C-C motif) ligand 2 (CCL2), and tumor necrosis factor α (TNFα). Synoviocyte survival was assessed by MTT assay. For the downregulation of HtrA2 transcripts, cells were transfected with HtrA2 siRNA.

RESULTS

We found that the concentration of HtrA2 was elevated in rheumatoid arthritis (RA) synovial fluid (SF) than in osteoarthritis (OA) SF, and its concentrations were correlated with the number of immune cells in the RA SF. Interestingly, HtrA2 levels in the SF of RA patients were elevated in proportion to synovitis severity and correlated with the expression of proinflammation cytokines and chemokines, such as IL-6, IL-8, and CCL2. In addition, HtrA2 was highly expressed in RA synovium and primary synoviocytes. RA synoviocytes released HtrA2 when stimulated with ER stress inducers. Knockdown of HtrA2 inhibited the IL1β-, TNFα-, and LPS-induced release of proinflammatory cytokines and chemokines by RA synoviocytes.

CONCLUSION

HtrA2 is a novel inflammatory mediator and a potential target for the development of an anti-inflammation therapy for RA.

Serine protease HtrA2/Omi regulates adaptive mitochondrial reprogramming in the brain cortex after ischemia/reperfusion injury via UCP2-SIRT3-PGC1 axis

This study is to investigate the underlying mechanisms of mitochondrial quality control (MQC) regulated by HtrA2/Omi during ischemia/reperfusion (I/R). We utilized the mnd2 mouse model, which has a missense mutation in HtrA2/Omi, to investigate the HtrA2/Omi regulation in mitochondria after I/R injury in the cerebral cortex. Compared to homozygous (HtrA2^mnd2) mice, heterozygous (HtrA2^Hetero) mice showed aging signs at a later age, increased HtrA2/Omi expression in the brain cortex, and lesser neurodegenerative signs. The brain cortex of HtrA2^Hetero mice had increased superoxide dismutase (SOD) activity; lower levels of malondialdehyde (MDA); higher expressions of mitochondrial unfolded protein response (mtUPR)-related proteins, NADH dehydrogenase [ubiquinone] iron-sulfur protein 7 (Ndufs7), and uncoupling protein 2 (UCP2) proteins; more mitochondrial fission; higher levels of ATP and mtDNA copies; elevated sirtuin 3 (SIRT3) activity; and increased NAD⁺/NADH ratio. After 1.5 h of I/R, the brain cortex of HtrA2^Hetero mice had a larger infarction size, reduced HtrA2/Omi expression, decreased S-X-linked inhibitor of apoptosis protein (XIAP), and increased C-Caspase3 than that of wild-type animals (WT). Mitochondria from the HtrA2^Hetero brain cortex showed decreased ATP production and MQC deficiency after 1.5 h I/R. Genipin pre-treatment reduced the aforementioned I/R injury in the HtrA2^Hetero brain cortex. In conclusion, mitochondrial function is compensated in the HtrA2^Hetero brain cortex via the upregulation of the UCP2-SIRT3-PGC1 axis. Decreased HtrA2/Omi function damages mitochondrial quality in the HtrA2^Hetero mouse brain cortex, leading to more brain I/R injury. Genipin pre-treatment ameliorates brain damages via the mitochondrial UCP2-SIRT3-PGC1 axis.

Omi inhibition ameliorates neuron apoptosis and neurological deficit after subarachnoid hemorrhage in rats

Background

Subarachnoid hemorrhage (SAH) is a severe neurological emergency, resulting in cognitive impairments and threatening human's health. Currently, SAH has no effective treatment. It is urgent to search for an effective therapy for SAH.

Objective

To explore the expression of Omi protein after subarachnoid hemorrhage in rats.

Methods

SAH rat model was established by injecting blood into the prechiasmatic cistern. Neurological deficit was assessed by detecting neurological deficit scores and brain tissue water contents. Apoptotic cells were evaluated by TUNEL staining and IHC staining. Omi and Cleaved caspase 3 expressions in nerve cells were determined by double staining using IF. Apoptosis-related proteins were measured by Western blotting assay.

Results

SAH rat model was successfully established, showing more apoptotic cells and high neurological deficit scores in SAH rat. In SAH rat model, Omi expression in nerve cells was elevated and the upregulation of Omi mainly occurred in cytoplasm, accompanied by the degradation of XIAP and the increased cleaved caspase 3/9 and cleaved PARP. Once treated with UCF-101, a specific inhibitor of Omi, the increased cell apoptosis, left/right brain moisture contents and neurological deficits were notably reversed in SAH rat brain. Of note, SAH-induced the increases of apoptosis-related protein in nerve cells were also rescued by the administration of UCF-101.

Conclusions

UCF-101-mediated Omi inhibition decreased the degradation of XIAP and subsequently inhibited the activation of apoptosis-related proteins, decreased nerve cell apoptosis, leading to the improvement on early brain injury in SAH rat. UCF-101-based Omi inhibition may be used to treat SAH with great potential application.

Progress in research on the role of Omi/HtrA2 in neurological diseases

Omi/HtrA2 is a serine protease present in the mitochondrial space. When stimulated by external signals, HtrA2 is released into the mitochondrial matrix where it regulates cell death through its interaction with apoptotic and autophagic signaling pathways. Omi/HtrA2 is closely related to the pathogenesis of neurological diseases, such as neurodegeneration and hypoxic ischemic brain damage. Here, we summarize the biological characteristics of Omi/HtrA2 and its role in neurological diseases, which will provide new hints in developing Omi/HtrA2 as a therapeutic target for neurological diseases.

Increased Active OMI/HTRA2 Serine Protease Displays a Positive Correlation with Cholinergic Alterations in the Alzheimer's Disease Brain

OMI/HTRA2 (high-temperature requirement serine protease A2) is a mitochondrial serine protease involved in several cellular processes, including autophagy, chaperone activity, and apoptosis. Few studies on the role of OMI/HTRA2 in Alzheimer's disease (AD) are available, but none on its relationship with the cholinergic system and neurotrophic factors as well as other AD-related proteins. In this study, immunohistochemical analyses revealed that AD patients had a higher cytosolic distribution of OMI/HTRA2 protein compared to controls. Quantitative analyses on brain extracts indicated a significant increase in the active form of OMI/HTRA2 in the AD brain. Activated OMI/HTRA2 protein positively correlated with stress-associated read-through acetylcholinesterase activity. In addition, α7 nicotinic acetylcholine receptor gene expression, a receptor also known to be localized on the outer membrane of mitochondria, showed a strong correlation with OMI/HTRA2 gene expression in three different brain regions. Interestingly, the activated OMI/HTRA2 levels also correlated with the activity of the acetylcholine-biosynthesizing enzyme, choline acetyltransferase (ChAT); with levels of the neurotrophic factors, NGF and BDNF; with levels of the soluble fragments of amyloid precursor protein (APP); and with gene expression of the microtubule-associated protein tau in the examined brain regions. Overall, the results demonstrate increased levels of the mitochondrial serine protease OMI/HTRA2, and a coherent pattern of association between the activated form of OMI/HTRA2 and several key proteins involved in AD pathology. In this paper, we propose a new hypothetical model to highlight the importance and needs of further investigation on the role of OMI/HTRA2 in the mitochondrial function and AD.

Down-regulated long non-coding RNA ANRIL restores the learning and memory abilities and rescues hippocampal pyramidal neurons from apoptosis in streptozotocin-induced diabetic rats via the NF-κB signaling pathway

Diabetes often causes learning and memory deficits, which leads to unfavorable behavioral performance. In this study, we investigated the effects of long non-coding RNA (lncRNA) ANRIL on learning, memory abilities, and hippocampal neuronal apoptosis via the NF-κB signaling pathway in streptozotocin (STZ)-induced diabetic rats. After successful establishment of diabetic rat models, the subjects were then assigned into the DM, DM + si-ANRIL, DM + si-negative control (si-NC) groups, as well as an additional normal group. Morris water maze test was employed to assess behavioral performance of rats, followed by the recording of body weight and blood glucose levels. Expressions of ANRIL, NF-κB signaling pathway-related, and apoptosis-related genes were examined by qRT-PCR and western blotting. Rat hippocampus expression levels of cleaved-caspase-3 were determined by immunofluorescence. Cell apoptosis was examined by TUNEL assay. Versus to the normal group, revealed there to be activation of the NF-κB signaling pathway, decreased weight, increased blood glucose, increased escape latency, reduced residence time, memory impairment, increased cleaved-caspase-3 expression, and increased apoptosis were detected in the DM and DM + si-NC groups. The DM + si-ANRIL group exhibited inhibited NF-κB signaling pathway, weight loss, decreased blood glucose, recovered memory, decreased cleaved-caspase-3 expression and reduced apoptosis compared to the DM group, with higher weight of rats, lower blood glucose levels, and stronger memory abilities in the DM + si-ANRIL group. Taken together, these findings indicate that silencing lncRNA ANRIL promotes memory recovery and decreases hippocampal neurons apoptosis in diabetic rats through the inhibition of the NF-κB signaling pathway.

Mitochondria mediates caspase-dependent and independent retinal cell death in Staphylococcus aureus endophthalmitis

Bacterial endophthalmitis, a vision-threatening complication of ocular surgery or trauma, is characterized by increased intraocular inflammation and retinal tissue damage. Although significant vision loss in endophthalmitis has been linked to retinal cell death, the underlying mechanisms of cell death remain elusive. In this study, using a mouse model of Staphylococcus aureus endophthalmitis and cultured human retinal Müller glia (MIO-M1 cell line), we demonstrate that S. aureus caused significant apoptotic cell death in the mouse retina and Müller glia, as evidenced by increased number of terminal dUTP nick end labeling and Annexin V and propidium iodide-positive cells. Immunohistochemistry and western blot studies revealed the reduction in mitochondrial membrane potential (JC-1 staining), release of cytochrome c into the cytosol, translocation of Bax to the mitochondria and the activation of caspase-9 and -3 in S. aureus-infected retina/retinal cells. In addition, the activation of PARP-1 and the release of apoptosis inducing factor from mitochondria was also observed in S. aureus-infected retinal cells. Inhibition studies using pan-caspase (Q-VD-OPH) and PARP-1 (DPQ) inhibitors showed significant reduction in S. aureus-induced retinal cell death both in vivo and in vitro. Together, our findings demonstrate that in bacterial endophthalmitis, retinal cells undergo apoptosis in the both caspase-dependent and independent manners, and mitochondria have a central role in this process. Hence, targeting the identified signaling pathways may provide the rationale to design therapeutic interventions to prevent bystander retinal tissue damage in bacterial endophthalmitis.

Matrix metalloproteinase-3 is activated by HtrA2/Omi in dopaminergic cells: relevance to Parkinson's disease

Dopaminergic neurons in the substantia nigra are particularly vulnerable, and their degeneration leads to Parkinson's disease. We have previously reported that matrix metalloproteinase-3 (MMP-3) activity is involved in dopaminergic neurodegeneration by multiple mechanisms and that this requires activation of MMP-3 from proMMP-3 by an intracellular serine protease. HtrA2/Omi is a mitochondrial serine protease that has been shown in non-dopaminergic cells to translocate into the cytosol where it triggers apoptosis. In the present study we sought to determine whether HtrA2/Omi might cause activation of MMP-3 in dopaminergic neuronal cells using CATH.a cell line. Mitochondrial stress induced by rotenone led to MMP-3 activation and HtrA2/Omi translocation into the cytosol. The MMP-3 activation involved HtrA2/Omi, because both pharmacological inhibition and siRNA-induced knockdown of HtrA2/Omi attenuated the activation induced by rotenone or MPP+. Overexpression of mature HtrA2/Omi, but not mutant HtrA2/Omi, resulted in MMP-3 activity increase and cell death. Addition of recombinant and catalytically active HtrA2/Omi to lysate of untreated cells led to activation of the endogenous MMP-3, and incubation of the HtrA2/Omi with recombinant proMMP-3 caused cleavage of proMMP-3 to a 48kD protein, corresponding to the active form, which was accompanied by an increase in MMP-3 activity. Taken together, the data indicate that HtrA2/Omi, which normally exists in the mitochondria, can cause MMP-3 activation in the cytosol under a cell stress condition, which can ultimately lead to demise of dopaminergic neuronal cells.