Proteolysis of oxidized proteins after oxygen-glucose deprivation in rat cortical neurons is mediated by the proteasome

Chronic traumatic encephalopathy-integration of canonical traumatic brain injury secondary injury mechanisms with tau pathology

In recent years, a new neurodegenerative tauopathy labeled Chronic Traumatic Encephalopathy (CTE), has been identified that is believed to be primarily a sequela of repeated mild traumatic brain injury (TBI), often referred to as concussion, that occurs in athletes participating in contact sports (e.g. boxing, American football, Australian football, rugby, soccer, ice hockey) or in military combatants, especially after blast-induced injuries. Since the identification of CTE, and its neuropathological finding of deposits of hyperphosphorylated tau protein, mechanistic attention has been on lumping the disorder together with various other non-traumatic neurodegenerative tauopathies. Indeed, brains from suspected CTE cases that have come to autopsy have been confirmed to have deposits of hyperphosphorylated tau in locations that make its anatomical distribution distinct for other tauopathies. The fact that these individuals experienced repetitive TBI episodes during their athletic or military careers suggests that the secondary injury mechanisms that have been extensively characterized in acute TBI preclinical models, and in TBI patients, including glutamate excitotoxicity, intracellular calcium overload, mitochondrial dysfunction, free radical-induced oxidative damage and neuroinflammation, may contribute to the brain damage associated with CTE. Thus, the current review begins with an in depth analysis of what is known about the tau protein and its functions and dysfunctions followed by a discussion of the major TBI secondary injury mechanisms, and how the latter have been shown to contribute to tau pathology. The value of this review is that it might lead to improved neuroprotective strategies for either prophylactically attenuating the development of CTE or slowing its progression.

Circulating proteasome activity following mild head injury in children

PURPOSE

The aim of the study is to characterize changes in circulating proteasome (c-proteasome) activity following mild traumatic brain injury in children.

METHODS

Fifty children managed at the Department of Pediatric Surgery because of concussion-mild head injury was randomly included into the study. The children were aged 11 months to 17 years (median = 10.07 + -1.91 years). Plasma proteasome activity was assessed using Suc-Leu-Leu-Val-Tyr-AMC peptide substrate, 2-6 h, 12-16 h, and 2 days after injury. Twenty healthy children admitted for planned inguinal hernia repair served as controls.

RESULTS

Statistically significant elevation of plasma c-proteasome activity was noted in children with mild head injury 2-6 h, 12-16 h, and 2 days after the injury.

CONCLUSIONS

Authors observed a statistically significant upward trend in the c-proteasome activity between 2-6 and 12-16 h after the mild head injury, consistent with the onset of the symptoms of cerebral concussion and a downward trend in the c-proteasome activity in the plasma of children with mild head injury between 12-16 h and on the second day after the injury, consistent with the resolving of the symptoms of cerebral concussion. Further studies are needed to demonstrate that the proteasome activity could be a prognostic factor, which can help in further diagnostic and therapeutic decisions in patients with head injury.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

PRAS40 is an integral regulatory component of erythropoietin mTOR signaling and cytoprotection

Emerging strategies that center upon the mammalian target of rapamycin (mTOR) signaling for neurodegenerative disorders may bring effective treatment for a number of difficult disease entities. Here we show that erythropoietin (EPO), a novel agent for nervous system disorders, prevents apoptotic SH-SY5Y cell injury in an oxidative stress model of oxygen-glucose deprivation through phosphatidylinositol-3-kinase (PI 3-K)/protein kinase B (Akt) dependent activation of mTOR signaling and phosphorylation of the downstream pathways of p70 ribosomal S6 kinase (p70S6K), eukaryotic initiation factor 4E-binding protein 1 (4EBP1), and proline rich Akt substrate 40 kDa (PRAS40). PRAS40 is an important regulatory component either alone or in conjunction with EPO signal transduction that can determine cell survival through apoptotic caspase 3 activation. EPO and the PI 3-K/Akt pathways control cell survival and mTOR activity through the inhibitory post-translational phosphorylation of PRAS40 that leads to subcellular binding of PRAS40 to the cytoplasmic docking protein 14-3-3. However, modulation and phosphorylation of PRAS40 is independent of other protective pathways of EPO that involve extracellular signal related kinase (ERK 1/2) and signal transducer and activator of transcription (STAT5). Our studies highlight EPO and PRAS40 signaling in the mTOR pathway as potential therapeutic strategies for development against degenerative disorders that lead to cell demise.

Glucose deficiency reduces collagen synthesis in breast cancer MCF7 cells

We decided to study the effect of glucose deprivation on collagen metabolism in MCF7 cells. The incorporation of [3H]-proline into collagenase-sensitive and hydroxyproline-containing proteins was used as an index of collagen synthesis, whereas pulse-chase technique was employed to evaluate the degradation of newly synthesized proteins. The MCF7 cells incubated in high glucose medium synthesized detectable amounts of collagenous proteins. Most of them were found in the cell layer. The shortage of glucose resulted in about 30% reduction in collagen synthesis. The pulse-chase experiments demonstrated that proportionally less collagen was degraded in cultures incubated in low-glucose than in high-glucose media.

Oxidative-stress-induced alterations in Sp factors mediate transcriptional regulation of the NR1 subunit in hippocampus during hypoxia

Ascent to high altitude is associated with tissue hypoxia resulting from the decrease in partial pressure of atmospheric oxygen. The hippocampus, in particular, is highly vulnerable to hypoxic insult, which at least in part can be attributed to the occurrence of glutamate excitotoxicity. Although this excitotoxic damage is often related to increased NMDA receptor activation and subsequent calcium-mediated free radical generation, the mechanisms involving the transcriptional regulation of NMDA receptor subunit expression by hypoxic stress remains to be explored. Our study reveals a novel mechanism for the regulation of expression of the NR1 subunit of NMDA receptors by the Sp family of transcription factors through an oxidative-stress-mediated mechanism that also involves the molecular chaperone Hsp90. The findings not only show the occurrence of lipid peroxidation and DNA damage in hippocampal cells exposed to hypoxia but also reveal a calcium-independent mechanism of selective oxidation and degradation of Sp3 by the 20S proteasome. This along with increased DNA binding activity of Sp1 leads to NR1 upregulation in the hippocampus during hypoxic stress. The study therefore provides evidence for free radical-mediated regulation of gene expression in hypoxia and the scope of the use of antioxidants in preventing excitotoxic neuronal damage during hypoxia.

Alterations of cerebral cortex and hippocampal proteasome subunit expression and function in a traumatic brain injury rat model

Following cellular stress or tissue injury, the proteasome plays a critical role in protein degradation and signal transduction. The present study examined the beta-subunit expression of constitutive proteasomes (beta1, beta2, and beta5), immunoproteasomes (beta1i, beta2i, and beta5i) and the 11S proteasome activator, PA28alpha, in the rat CNS after traumatic brain injury (TBI). Concomitant measures assessed changes in proteasome activities. Quantitative real time PCR results indicated that beta1 and beta2 mRNA levels were not changed, while beta5 mRNA levels were significantly decreased in injured CNS following TBI. However, beta1i, beta2i, beta5i, and PA28alpha mRNA levels were significantly increased in the injured CNS. Western blotting studies found that beta1, beta2, beta5, beta2i, and beta5i subunit protein levels remained unchanged in the injured CNS, but beta1i and PA28alpha protein levels were significantly elevated in ipsilateral cerebral cortex and hippocampus. Proteasome activity assays found that peptidyl glutamyl peptide hydrolase-like and chymotrypsin-like activity were significantly reduced in the CNS after TBI, and that trypsin-like proteasome activity was increased in the injured cerebral cortex. Our results demonstrated that both proteasome composition and function in the CNS were affected by trauma. Treatments that preserve proteasome function following CNS injury may be beneficial as an approach to cerebral neuroprotection.

Ubiquitin and ubiquitin-conjugated protein expression in the rat cerebral cortex and hippocampus following traumatic brain injury (TBI)

Regulation of protein turnover is essential to the survival of eukaryotic cells. This important cellular process is partly regulated by the ubiquitin-proteasome system through posttranslational modification by the conjugation of ubiquitin chains to proteins targeted for degradation by proteasomes. The present study examined ubiquitin mRNA and protein expression in the CNS of rats that sustained traumatic brain injury (TBI). Quantitative real-time polymerase chain reaction results indicated that mRNA levels of ubA52, ubB and ubC in the ipsilateral cerebral cortex were significantly decreased on Day 1 post-TBI, that ubC mRNA levels also were significantly lower than control on Day 3 post-TBI, but that by Day 7 post-TBI, ubA52, ubB and ubC mRNA levels had all returned to control levels. In the ipsilateral hippocampus, ubA52 mRNA levels were significantly lower on Days 1-7 post-TBI, while ubB and ubC mRNA levels were less only on Day 1 post-TBI. Western blotting found that free ubiquitin protein levels were significantly reduced in both ipsilateral cerebral cortex and hippocampus on Days 1-7 post-TBI, while there was markedly increased ubiquitin-conjugated protein in ipsilateral cerebral cortex on Day 7 and in hippocampus on Days 3-7 post-TBI. Our study suggests that altered ubiquitin system function in the CNS contributes to the pathological outcomes of TBI.

Glucose-depleted medium reduces the collagen content of human skin fibroblast cultures

Glucose deprivation appeared to be a factor which induces oxygen regulated protein (ORP) 150 expression in the human skin fibroblasts cultures. Whereas glucose deprivation resulted in a slight (statistically insignificant) decrease of protein content in these cultures, a marked decrease of collagen content was observed, resulting in a distinct reduction of hydroxyproline: protein ratio. Furthermore, the appearance of ORP150 in glucose-deprived cultures coexisted with an increase of gelatinolytic activity and slight reduction in the expression of insulin-like growth factor-I (IGF-I) receptor. Since IGF-I is a main stimulator of collagen synthesis, the reduction in the expression of IGF-I receptor may result in a decrease of collagen synthesis. It is suggested that ORP 150 is a chaperon, which protects intracellular proteins against proteolytic effects exerted by hypoxia or glucose shortage. Since the total amount of protein in fibroblast cultures did not change much, it appears that collagen (in contrast to other proteins) was not efficiently protected. The decrease in collagen synthesis and the enhancement of collagen degradation by gelatinases may result in distinct reduction of collagen content in glucose-deprived fibroblast cultures.

Proteasome inhibition protects HT22 neuronal cells from oxidative glutamate toxicity

Oxidative stress caused by glutathione depletion after prolonged exposure to extracellular glutamate leads to a form of neuronal cell death that exhibits morphologically mixed features of both apoptosis and necrosis. However, specific downstream executioners involved in this form of cell death have yet to be identified. We report here that glutamate exposure does not activate caspase-3 in the HT22 neuronal cell line. Furthermore, no cytoprotection was achieved with either the pan-caspase inhibitor Z-VAD-fmk or the caspase-3-specific inhibitor DEVD-CHO. In contrast, inhibition of the proteasome by lactacystin protected both HT22 cells and rat primary neuronal cells against cell lysis. In parallel, oxidatively altered and ubiquitinated proteins accumulated in the mitochondrial fraction of cells after proteasome inhibition. These findings suggest that caspases can be decoupled from oxidative stress under some conditions, and implicate the ubiquitin/proteasome pathway in neuronal cell death caused by oxidative glutamate toxicity.

Iron uptake of the normoxic, anoxic and postanoxic microglial cell line RAW 264.7

Iron is one of the trace elements playing a key role in the normal brain metabolism. An excess of free iron on the other hand is catalyzing the iron-mediated oxygen radical production. Such a condition might be a harmful event leading perhaps to serious tissue damage and degeneration. Therefore, during evolution a complex iron sequestering apparatus developed, minimizing the amount of redox-reactive free iron. However, this system might be severely disturbed under pathophysiological conditions including hypoxia or anoxia. Since little is known about the non-transferrin-mediated iron metabolism of the brain during anoxia/reoxygenation, we tested the ability of the microglial cell line RAW 264.7 to take up iron independently of transferrin under various oxygen concentrations. Microglial cells are thought to be the major player in the maintenance of the extracellular homeostasis in the brain. Therefore, we investigated the iron metabolism of microglial cells employing radiolabeled ferric chloride. We tested the uptake of iron under normoxic, anoxic and postanoxic conditions. Furthermore, the amount of ferritin was measured by immunoblotting. We were able to show that iron enters the microglial cell line in the absence of extracellular transferrin under normoxic, anoxic and postanoxic conditions. Interestingly, the amount of ferritin is decreasing in the early reoxygenation phase. Therefore, we concluded that microglia is able to contribute to the brain iron homeostasis under anoxic and postanoxic conditions.

Protein oxidation and the degradation of oxidized proteins in the rat oligodendrocyte cell line OLN 93-antioxidative effect of the intracellular spin trapping agent PBN

Oligodendrocytes are the myelin-producing cells in the central nervous system. It was proposed that these cells are much more prone to oxidative damage than to other cells of the central nervous system. This fact seems to be due to their high iron store and low antioxidative defense mechanisms. Consequently, free radical induced damage should lead to an enhanced damage of oligodendrocytes. Thus, we chose the oligodendrocyte cell line OLN 93 to measure the stability of the protein pool after oxidation and the possibilities of protecting proteins by alpha-phenyl-N-tert-butylnitrone (PBN). We were able to demonstrate for the first time that OLN 93 cells are able to respond with an increase in overall proteolysis when exposed to various oxidants. This increase was the consequence of an enhanced protein oxidation. The activity of the 20S proteasome, which is thought to be involved in the removal of oxidized proteins, was not effected by moderate concentrations of the oxidants. The spin-trap PBN was used as an antioxidant and was able to prevent protein oxidation in OLN 93 cells effectively. Consequently, we proved that PBN is also able to prevent the increase in overall protein oxidation. We were able to demonstrate that OLN 93 oligodendrocytes react to oxidative stress with an increase in the protein turnover directed towards the removal of oxidized proteins. The intracellular spin-trap PBN is able to prevent protein oxidation in OLN 93 cells.

Antioxidants effectively prevent oxidation-induced protein damage in OLN 93 cells

Oxidative stress is supposed to play an important role in demyelinating diseases. Oligodendrocytes are the myelin-forming cells in the brain and are highly susceptible to oxidative stress due to their low antioxidative defense systems and high metabolic rate. In the present work, we tested the response of the oligodendrocyte cell line OLN 93 to oxidative stress. OLN 93 cell cultures are characterized by a loss of cell viability after oxidation. This loss of cell viability is accompanied by an increase in protein oxidation and consequently an elevated overall proteolysis. To minimize the oxidative damage, we tested the effects of the antioxidants alpha-lipoic acid and coenzyme Q(10). Both compounds were able to elevate cell viability and to decrease intracellular protein turnover and oxidant induced protein oxidation. Therefore, we concluded that the excessive oxidative damage of oligodendrocytes and their protein pool can be prevented by the usage of antioxidants.

Alterations of cullin-5 mRNA levels in the rat central nervous system following hemorrhagic shock

Hemorrhagic shock is a clinical syndrome that manifests as hypoperfusion, hypoxia, and ischemia initiating various cellular stress responses involved in the synthesis and release of an assortment of pro-inflammatory molecules, cytokines, chemokines, and reactive oxidant species (ROS). The ROS have been shown to oxidize and damage proteins making them targets for ubiquitination and proteasomal degradation. Cullin-5 (cul-5), an E3 ligase that binds ubiquitin to proteins targeted for degradation via the proteasome, was investigated for its gene expression during hemorrhagic shock. Male Long-Evans rats were subjected to volume controlled (27 ml kg-1) hemorrhage over 10 min and kept in shock for 60 min. Quantitative realtime polymerase chain reaction showed cul-5 mRNA levels were significantly increased in the brainstem and cerebellum, and decreased in the hypothalamus of rats as a result of hemorrhagic shock (n = 6) compared to sham-treated rats (n = 6). Cul-5 mRNA levels in the cerebral cortex, small intestine, kidney, liver, lung, or pituitary gland did not significantly change after hemorrhagic shock. This is the first report of cul-5 mRNA regulation by hemorrhagic shock. Evidence indicates this protein may have a regulatory role in ubiquitin-proteasomal protein degradation in response to hemorrhagic shock.