c-Jun-dependent sulfiredoxin induction mediates BDNF protection against mitochondrial inhibition in rat cortical neurons

BDNF specifically expressed in hippocampal neurons is involved in methylmercury neurotoxicity resistance

Methylmercury (MeHg) causes selective neuronal damage to cerebrocortical neurons (CCNs) in the central nervous system, but not to hippocampal neurons (HiNs), which are highly vulnerable to neurodegenerative diseases. In our previous study using cultured rat neurons, we performed a comprehensive gene expression analysis and found that the brain-derived neurotrophic factor (BDNF), a neurotrophin (NT), was specifically expressed in HiNs. Therefore, to elucidate the causal factors of MeHg toxicity resistance in HiNs, we conducted a comparative study of the protein expression and function of several NTs, including BDNF, using CCNs showing vulnerability to MeHg toxicity and HiNs showing resistance. BDNF was specifically expressed in HiNs, whereas nerve growth factor was barely detectable in either neuron type. In addition, other NTs, NT3 and NT4/5, were expressed in small but nearly equal amounts in both neuron types. Furthermore, among the various pathways involved in MeHg neurotoxicity, the p44/42 MAPK pathway was specifically activated in HiNs, even without MeHg treatment. siRNAs were used to reduce NTs in both neuron types. Only a specific reduction in BDNF attenuated the resistance to MeHg toxicity and p44/42 MAPK activation in HiNs. In addition, the external addition of BDNF and NT4/5, which act on the same tyrosine receptor kinase (Trk), TrkB, suppressed MeHg neurotoxicity in both neuron types. These results suggest that BDNF, expressed specifically in HiNs, is involved in the resistance to MeHg neurotoxicity via TrkB. Additionally, the activation of the p44/42 MAPK pathway may contribute to the inhibitory effect of BDNF on MeHg neurotoxicity.

Identifying potential neuroprotective polyphenols targeting endoplasmic reticulum stress through an in silico approach

The endoplasmic reticulum (ER) is essential in many cellular processes, including protein processing, lipid metabolism, and calcium storage. Dysregulation of ER function has been linked with neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, etc. The primary pathological alteration explicated in the diseases is the accumulation of misfolded proteins in the neuronal cells. ER stress-associated activation of PERK-mediated pro-apoptotic cell death leads to neurodegeneration. In this study, we have primarily screened the potential polyphenols evidenced for neuroprotective activity. The 24 polyphenols were selected to explore their binding affinity towards various proteins of ER cascade such as pPERK (phospho-PERK), EIF2 (Eukaryotic Initiation Factor 2), and ATF4 (Activating Transcription Factor 4). On the basis of binding affinity, four phytopolyphenols were further selected for in-silico ADMET and molecular dynamic simulation. Among them curcumin found to be the most promising and serve as a potential hit against all three targets of ER cascade. The selected proteins' active site has demonstrated high stability of curcumin binding according to molecular dynamics findings. Though curcumin exhibited a significant hit in interaction with targets but needs to be further improved in drug-ability criteria. Thus, seventy derivatives of curcumin scaffold (from the published literature) were also screened with improve in druggability criteria, which showed good interaction with unfolded protein response related targets. The new scaffolds serve considerable potential to be developed as novel polyphenolic lead for neurodegenerative disorders.Communicated by Ramaswamy H. Sarma.

Lithium Benzoate Exerts Neuroprotective Effect by Improving Mitochondrial Function, Attenuating Reactive Oxygen Species, and Protecting Cognition and Memory in an Animal Model of Alzheimer’s Disease

Background:

Alzheimer’s disease (AD) is a multifactorial neurodegenerative disease affecting many cellular pathways, including protein aggregation, mitochondrial dysfunction, oxidative stress (OS), and neuroinflammation. Currently, no effective treatment for AD exists.

Objective:

We aim to determine the effect of lithium benzoate (LiBen) in protecting neurons from amyloid-β (Aβ) or other neurotoxin insults.

Methods:

Primary rat cortical neurons co-treated with neurotoxins and LiBen were used to examine its effect in cell viability, reactive oxygen species (ROS) clearance, and mitochondrial functions by MTT, CellRox fluorescence staining, and seahorse assay. Then, Barnes maze and prepulse inhibition test were performed in APP/PS1 mice that received chronic LiBen treatment to assess its effect on cognitive protection. Oral bioavailability of LiBen was also assessed by pharmacokinetic study in rat plasma.

Results:

In this study, we discovered that LiBen can attenuate cellular ROS level, improve mitochondrial function, increase cell viability against multiple different insults of mitochondrial dysfunction, Aβ accumulation, and neuroinflammation, and promote neurogenesis. We demonstrated that LiBen has advantages over lithium or sodium benzoate alone as LiBen displays superior neuroprotective efficacy and oral bioavailability than the other two agents when being applied either alone or in combination. Furthermore, chronic administration of LiBen showed protection for cognition as well as spatial memory and reduced the senile plaque deposition in brains of AD animal models.

Conclusion:

LiBen stands as a promising therapeutic agent for improving cognition and delaying the progression of AD.

Sulfiredoxin-1 blood mRNA expression levels negatively correlate with hippocampal atrophy and cognitive decline

Introduction: Cognitive decline, correlating with hippocampal atrophy, characterizes several neurodegenerative disorders having a background of low-level chronic inflammation and oxidative stress.

Methods: In this cross-sectional study, we examined how cognitive decline and hippocampal subfields volume are associated with the expression of redox and inflammatory genes in peripheral blood. We analyzed 34 individuals with different cognitive scores according to Mini-Mental State Examination, corrected by age and education (adjMMSE). We identified a group presenting cognitive decline (CD) with adjMMSE<27 (n=14) and a normal cognition (NC) group with adjMMSE≥27 (n=20). A multiparametric approach, comprising structural magnetic resonance imaging measurement of different hippocampal segments and blood mRNA expression of redox and inflammatory genes was applied.

Results: Our findings indicate that hippocampal segment volumes correlate positively with adjMMSE and negatively with the blood transcript levels of 19 genes, mostly redox genes correlating especially with the left subiculum and presubiculum. A strong negative correlation between hippocampal subfields atrophy and Sulfiredoxin-1 ( SRXN1) redox gene was emphasized.

Conclusions: Concluding, these results suggest that SRXN1 might be a valuable candidate blood biomarker for non-invasively monitoring the evolution of hippocampal atrophy in CD patients.

The p53/NF-kappaB-dependent induction of sestrin2 by amyloid-beta peptides exerts antioxidative actions in neurons

Accumulation of senile plaques mainly composed of neurotoxic amyloid-beta peptide (Aβ) is a pathological hallmark of Alzheimer's disease (AD). Sestrin2 inducible by various types of stressors is known to promote autophagy and exert antioxidative effects. In this work, we revealed the molecular mechanisms underlying Aβ induction of sestrin2 and tested whether antioxidation, in addition to autophagy regulation, also contributes to its neuroprotective effects in primary rat cortical neurons. We found that Aβ25-35 triggered nuclear translocation of p65 and p50, two subunits of nuclear factor-kappaB (NF-κB), and p53. Aβ25-35-induced sestrin2 expression was abolished by the p65 siRNA, the NF-κB inhibitor SN50, and the p53 inhibitor pifithrin-alpha (PFT-α). Further, Aβ25-35 enhanced binding of p50 and p53 to sestrin2 gene promoter that was abolished respectively by the p50 shRNA and PFT-α. Both p50 shRNA and PFT-α attenuated Aβ25-35-induced expression as well as nuclear translocation of all three transcription factors, namely p65, p50, and p53. Interestingly, p50 binding to the promoters of its target genes required p53 activity, whereas p50 also negatively regulated p53 binding to its target sequences. Suppression of sestrin2 expression by siRNA enhanced Aβ25-35- and Aβ1-42-induced production of reactive oxygen species (ROS), lipid peroxidation, and formation of 8-hydroxy-2-deoxyguanosine (8-OH-dG). In contrast, overexpression of the sestrin2 N-terminal or C-terminal fragments neutralized Aβ25-35-induced ROS production. We concluded that Aβ-induced sestrin2 contributing to antioxidant effects in neurons is in part mediated by p53 and NF-κB, which also mutually affect the expression of each other.

The Secretion from Bone Marrow Mesenchymal Stem Cells Pretreated with Berberine Rescues Neurons with Oxidative Damage Through Activation of the Keap1-Nrf2-HO-1 Signaling Pathway

Oxidative stress is a potential pathological mechanism of Alzheimer's disease (AD). Berberine (BBR) can improve antioxidative capacity and inhibit Aβ protein aggregation and tau protein hyperphosphorylation in AD, and stem cell therapy is also increasingly recognized as a therapy for AD. Bone marrow mesenchymal stem cells (BMSCs) have many advantages, as they exhibit antioxidant and anti-inflammatory activity and secrete a variety of neurotrophic factors, and play important roles in neurodegenerative disease treatment. In this study, we investigated the antioxidant effects of secretions from BMSCs pretreated with BBR on tert-butyl hydroperoxide (t-BHP)-damaged neurons. We demonstrated that BBR can enhance BMSC viability and the secretion of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), both of which are vital neurotrophic factors that maintain neuronal growth. Moreover, conditioned medium from BBR-treated BMSCs (BBR-BMSC-CM) reduced reactive oxygen species (ROS) production, attenuated a decrease in the mitochondrial membrane potential, and ameliorated neuronal apoptosis by decreasing levels of the apoptotic proteins Bax/Bcl-2, cytochrome c, and cleaved caspase-3/caspase-3. In addition, increased synaptophysin (SYP) and postsynaptic density protein 95 (PSD95) levels indicated that neuronal synaptic function was restored. Further study revealed that BBR-BMSC-CM activated the antioxidant proteins Keap1, Nrf2, and HO-1. In conclusion, our results showed that BBR-BMSC-CM attenuated apoptosis and oxidative damage in neurons by activating the Keap1-Nrf2-HO-1 signaling pathway. Taken together, these results also suggest BBR as a drug to stimulate the secretion of nutritional cytokines with the potential to treat AD.

Morphine consumption during pregnancy exacerbates neonatal hypoxia-ischemia injury in rats

OBJECTIVE

Hypoxia-Ischemia (HI) is the most common cause of death and disability in human infants. The use of opiate in pregnant women affects their children. The aim of this study was to evaluate the effect of morphine consumption during pregnancy and lactation on vulnerability to neonatal HI in rats.

MATERIALS AND METHODS

Female Wistar rats were randomly assigned into two groups: Group 1-Rats that did not receive any treatment during pregnancy and lactation and Group 2-Rats that received morphine during pregnancy and lactation. After delivery, male offspring were divided into four groups including: (a) SHAM, (b) SHAM/Morphine (SHAM/MO), (c) HI, (d) HI/Morphine (HI/MO). Seven days after HI induction, neurobehavioral tests were performed, and then, brain tissue was taken from the skull to measure cerebral edema, infarct volume, inflammatory factors, oxidative stress, and brain-derived neurotrophic factor (BDNF).

RESULTS

Total antioxidant capacity (TAC) and BDNF levels in the HI/MO group were significantly lower than HI and SHAM groups. TNF-α, C-reactive protein and total oxidant capacity levels in the HI/MO group were significantly higher than HI and SHAM groups. Cerebral edema and infarct volume in the HI/MO group were significantly higher than the HI group.

CONCLUSION

Based on the results, morphine consumption during pregnancy and lactation enhanced the deleterious effects of HI injury in pups.

The role of transglutaminase 2 in mediating glial cell function and pathophysiology in the central nervous system

The ubiquitously expressed transglutaminase 2 (TG2) has diverse functions in virtually all cell types, with its role depending not only on cell type, but also on specific subcellular localization. In the central nervous system (CNS) different types of glial cells, such as astrocytes, microglia, and oligodendrocytes and their precursor cells (OPCs), play pivotal supportive functions. This review is focused on what is currently known about the role of TG2 in each type of glial cell, in the context of normal function and pathophysiology. For example, astrocytic TG2 facilitates their migration and proliferation, but hinders their ability to protect neurons after CNS injury. The review also examines the interactions between glial cell types, and how TG2 in one cell type may affect another, as well as implications for specific TG2 populations as therapeutic targets in CNS pathology.

Inhibition of the Epigenetic Regulator REST Ameliorates Ischemic Brain Injury

Cerebral ischemia is known to activate the repressor element-1 (RE1)-silencing transcription factor (REST) which silences neural genes via epigenetic remodeling and promotes neurodegeneration. We presently determined if REST inhibition derepresses target genes involved in synaptic plasticity and promotes functional outcome after experimental stroke. Following transient focal ischemia induced by middle cerebral artery occlusion (MCAO) in adult rats, REST expression was upregulated significantly from 12 h to 1 day of reperfusion compared to sham control. At 1 day of reperfusion, REST protein levels were increased and observed in the nuclei of neurons in the peri-infarct cortex. REST knockdown by intracerebral REST siRNA injection significantly reduced the post-ischemic expression of REST and increased the expression of several REST target genes, compared to control siRNA group. REST inhibition also decreased post-ischemic markers of apoptosis, reduced cortical infarct volume, and improved post-ischemic functional recovery on days 5 and 7 of reperfusion compared to the control siRNA group. REST knockdown resulted in a global increase in synaptic plasticity gene expression at 1 day of reperfusion compared to the control siRNA group and significantly increased several synaptic plasticity genes containing RE-1 sequences in their regulatory regions. These results demonstrate that direct inhibition of the epigenetic remodeler REST prevents secondary brain damage in the cortex and improves functional outcome potentially via de-repression of plasticity-related genes after stroke.

Sodium selenite protects from 3-nitropropionic acid-induced oxidative stress in cultured primary cortical neurons

Selenium (Se) is an essential trace element for humans; its intake is needed to allow the proper synthesis of 25 different selenoproteins that are necessary to the normal functioning of several organs, including the brain. Accordingly, decreased Se levels have been associated with neurological disorders. In the present study, we investigated the potential beneficial effects of Se, as sodium selenite, against 3-nitropropionic acid (3-NP)-induced oxidative stress in primary cultures of mouse cortical neurons. 3-NP treatment caused a significant decrease in cellular viability, which was accompanied by decreases in mitochondrial complex II activity and reduced glutathione (GSH) content, as well as increases in reactive oxygen species (ROS) generation and oxidized glutathione (GSSG) levels. Sodium selenite pretreatment (6 days) attenuated 3-NP-induced decrease in cell viability. In addition, sodium selenite pretreatment significantly protected against 3-NP-induced increase in ROS generation and decrease in GSH/GSSG ratio. Of note, sodium selenite pretreatment did not change 3-NP-induced decrease of mitochondrial complex II activity, suggesting that Se modulates secondary events resultant from 3-NP-induced mitochondrial dyshomeostasis. In addition, sodium selenite pretreatment significantly increased glutathione peroxidase (GPx) activity. Our data provide insights into the mechanism of protection by sodium selenite, which is related, at least in part, to GPx induction.

Towards frailty biomarkers: Candidates from genes and pathways regulated in aging and age-related diseases

OBJECTIVE

Use of the frailty index to measure an accumulation of deficits has been proven a valuable method for identifying elderly people at risk for increased vulnerability, disease, injury, and mortality. However, complementary molecular frailty biomarkers or ideally biomarker panels have not yet been identified. We conducted a systematic search to identify biomarker candidates for a frailty biomarker panel.

METHODS

Gene expression databases were searched (http://genomics.senescence.info/genes including GenAge, AnAge, LongevityMap, CellAge, DrugAge, Digital Aging Atlas) to identify genes regulated in aging, longevity, and age-related diseases with a focus on secreted factors or molecules detectable in body fluids as potential frailty biomarkers. Factors broadly expressed, related to several "hallmark of aging" pathways as well as used or predicted as biomarkers in other disease settings, particularly age-related pathologies, were identified. This set of biomarkers was further expanded according to the expertise and experience of the authors. In the next step, biomarkers were assigned to six "hallmark of aging" pathways, namely (1) inflammation, (2) mitochondria and apoptosis, (3) calcium homeostasis, (4) fibrosis, (5) NMJ (neuromuscular junction) and neurons, (6) cytoskeleton and hormones, or (7) other principles and an extensive literature search was performed for each candidate to explore their potential and priority as frailty biomarkers.

RESULTS

A total of 44 markers were evaluated in the seven categories listed above, and 19 were awarded a high priority score, 22 identified as medium priority and three were low priority. In each category high and medium priority markers were identified.

CONCLUSION

Biomarker panels for frailty would be of high value and better than single markers. Based on our search we would propose a core panel of frailty biomarkers consisting of (1) CXCL10 (C-X-C motif chemokine ligand 10), IL-6 (interleukin 6), CX3CL1 (C-X3-C motif chemokine ligand 1), (2) GDF15 (growth differentiation factor 15), FNDC5 (fibronectin type III domain containing 5), vimentin (VIM), (3) regucalcin (RGN/SMP30), calreticulin, (4) PLAU (plasminogen activator, urokinase), AGT (angiotensinogen), (5) BDNF (brain derived neurotrophic factor), progranulin (PGRN), (6) α-klotho (KL), FGF23 (fibroblast growth factor 23), FGF21, leptin (LEP), (7) miRNA (micro Ribonucleic acid) panel (to be further defined), AHCY (adenosylhomocysteinase) and KRT18 (keratin 18). An expanded panel would also include (1) pentraxin (PTX3), sVCAM/ICAM (soluble vascular cell adhesion molecule 1/Intercellular adhesion molecule 1), defensin α, (2) APP (amyloid beta precursor protein), LDH (lactate dehydrogenase), (3) S100B (S100 calcium binding protein B), (4) TGFβ (transforming growth factor beta), PAI-1 (plasminogen activator inhibitor 1), TGM2 (transglutaminase 2), (5) sRAGE (soluble receptor for advanced glycosylation end products), HMGB1 (high mobility group box 1), C3/C1Q (complement factor 3/1Q), ST2 (Interleukin 1 receptor like 1), agrin (AGRN), (6) IGF-1 (insulin-like growth factor 1), resistin (RETN), adiponectin (ADIPOQ), ghrelin (GHRL), growth hormone (GH), (7) microparticle panel (to be further defined), GpnmB (glycoprotein nonmetastatic melanoma protein B) and lactoferrin (LTF). We believe that these predicted panels need to be experimentally explored in animal models and frail cohorts in order to ascertain their diagnostic, prognostic and therapeutic potential.

Depletion of astrocytic transglutaminase 2 improves injury outcomes

Astrocytes play an indispensable role in maintaining a healthy, functional neural network in the central nervous system (CNS). A primary function of CNS astrocytes is to support the survival and function of neurons. In response to injury, astrocytes take on a reactive phenotype, which alters their molecular functions. Reactive astrocytes have been reported to be both beneficial and harmful to the CNS recovery process subsequent to injury. Understanding the molecular processes and regulatory proteins that determine the extent to which an astrocyte hinders or supports neuronal survival is important within the context of CNS repair. One protein that plays a role in modulating cellular survival is transglutaminase 2 (TG2). Global deletion of TG2 results in beneficial outcomes subsequent to in vivo ischemic brain injury. Ex vivo studies have also implicated TG2 as a negative regulator of astrocyte viability subsequent to injury. In this study we show that knocking down TG2 in astrocytes significantly increases their ability to protect neurons from oxygen glucose deprivation (OGD)/reperfusion injury. To begin to understand how deletion of TG2 in astrocytes improves their ability to protect neurons from injury, we performed transcriptome analysis of wild type and TG2^-/- astrocytes. TG2 deletion resulted in alterations in genes involved in extracellular matrix remodeling, cell adhesion and axon growth/guidance. In addition, the majority of genes that showed increases in the TG2^-/- astrocytes had predicted cJun/AP-1 binding motifs in their promoters. Furthermore, phospho-cJun levels were robustly elevated in TG2^-/- astrocytes, a finding which was consistent with the increase in expression of AP-1 responsive genes. These in vitro data were subsequently extended into an in vivo model to determine whether the absence of astrocytic TG2 improves outcomes after CNS injury. Our results show that, following a spinal cord injury, scar formation is significantly attenuated in mice with astrocyte-specific TG2 deletion compared to mice expressing normal TG2 levels. Taken together, these data indicate that TG2 plays a pivotal role in mediating reactive astrocyte properties following CNS injury. Further, the data suggest that limiting the AP-1 mediated pro-survival injury response may be a contributing factor to that the detrimental effects of astrocytic TG2.

Nrf2-activated expression of sulfiredoxin contributes to urethane-induced lung tumorigenesis

Lung cancer is the leading cause of cancer death worldwide. Cigarette smoking and exposure to chemical carcinogens are among the risk factors of lung tumorigenesis. In this study, we found that cigarette smoke condensate and urethane significantly stimulated the expression of sulfiredoxin (Srx) at the transcript and protein levels in cultured normal lung epithelial cells, and such stimulation was mediated through the activation of nuclear related factor 2 (Nrf2). To study the role of Srx in lung cancer development in vivo, mice with Srx wildtype, heterozygous or knockout genotype were subjected to the same protocol of urethane treatment to induce lung tumors. By comparing tumor multiplicity and volume between groups of mice with different genotype, we found that Srx knockout mice had a significantly lower number and smaller size of lung tumors. Mechanistically, we demonstrated that loss of Srx led to a decrease of tumor cell proliferation as well as an increase of tumor cell apoptosis. These data suggest that Srx may have an oncogenic role that contributes to the development of lung cancer in smokers or urethane-exposed human subjects.

Id1 and Sonic Hedgehog Mediate Cell Cycle Reentry and Apoptosis Induced by Amyloid Beta-Peptide in Post-mitotic Cortical Neurons

Amyloid beta-peptide (Aβ), the neurotoxic component of senile plaques in Alzheimer's disease (AD) brains, is known to trigger cell cycle reentry in post-mitotic neurons followed by apoptosis. However, the underlying mechanisms remain unclear. Recently, we have reported that Aβs stimulate the expression of inhibitor of differentiation-1 (Id1) to induce sonic hedgehog (SHH) (Hung et al., Mol Neurobiol 53(2):793-809, 2016), and both are mitogens capable of triggering cell cycle progression. In this work, we tested the hypothesis that Aβ-induced Id1 and SHH contribute to cell cycle reentry leading to apoptosis in neurons. We found that Aβ triggered cell cycle progression in the post-mitotic neurons, as indicated by the increased expression of two G1-phase markers including cyclin D1 and phosphorylated retinoblastoma protein (pRb), two G2-phase markers such as proliferating cell nuclear antigen (PCNA) and incorporation of 5-bromo-2'-deoxyuridine (BrdU) into newly synthesized DNA, as well as the mitotic marker histone H3 phosphorylated at Ser-10. As expected, Aβ also enhanced caspase-3 cleavage in the cortical neurons. Id1 siRNA, the neutralization antibody against SHH (SHH-Ab), and the cyclin-dependent kinase (CDK)-4/6 inhibitor PD0332991 all attenuated, in part or in full, the Aβ-induced expression of these cell cycle markers. Indeed, exogenous recombinant Id1 protein and the biologically active N-terminal fragment of SHH (SHH-N) were both sufficient to enhance the expression of cell cycle markers independent of Aβ. Taken together, our results revealed the critical roles of Id1 and SHH mediating Aβ-dependent cell cycle reentry and subsequently caspase-dependent apoptosis in the fully differentiated post-mitotic neurons, at least in vitro.

More Insight into BDNF against Neurodegeneration: Anti-Apoptosis, Anti-Oxidation, and Suppression of Autophagy

In addition to its well-established neurotrophic action, brain-derived neurotrophic factor (BDNF) also possesses other neuroprotective effects including anti-apoptosis, anti-oxidation, and suppression of autophagy. We have shown before that BDNF triggers multiple mechanisms to confer neuronal resistance against 3-nitropropionic acid (3-NP)-induced mitochondrial dysfunction in primary rat cortical cultures. The beneficial effects of BDNF involve the induction of anti-oxidative thioredoxin with the resultant expression of anti-apoptotic B-cell lymphoma 2 (Bcl-2) as well as erythropoietin (EPO)-dependent stimulation of sonic hedgehog (SHH). We further revealed that BDNF may bring the expression of sulfiredoxin, an ATP-dependent antioxidant enzyme, to offset mitochondrial inhibition in cortical neurons. Recently, we provided insights into another novel anti-oxidative mechanism of BDNF, which involves the augmentation of sestrin2 expression to endow neuronal resistance against oxidative stress induced by 3-NP; BDNF induction of sestrin2 entails the activation of a pathway involving nitric oxide (NO), cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG), and nuclear factor-κB (NF-κB). Apart from anti-apoptosis and anti-oxidation, we demonstrated in our most recent study that BDNF may activate the mammalian target of rapamycin (mTOR) with resultant activation of transcription factor c-Jun, thereby stimulating the expression of p62/sequestosome-1 to suppress heightened autophagy as a result of 3-NP exposure. Together, our results provide in-depth insight into multi-faceted protective mechanisms of BDNF against mitochondrial dysfunction commonly associated with the pathogenesis of many chronic neurodegenerative disorders. Delineation of the protective signaling pathways elicited by BDNF would endow a rationale to develop novel therapeutic regimens to halt or prevent the progression of neurodegeneration.

Sulfiredoxin involved in the protection of peroxiredoxins against hyperoxidation in the early hyperglycaemia

As a direct consequence of hyperglycaemia, the excessive generation of ROS is central to the pathogenesis of diabetic cardiomyopathy. We hypothesize that stimulation of high glucose (HG) results in an increased sulfiredoxin (Srx) expression, which regulates ROS signaling through reducing the hyperoxidized peroxiredoxins (Prxs). We show that hyperoxidized Prxs were initially reduced in the preliminary stage but then dramatically increased in advanced stage and these changes corresponded to a significant increase of Srx expression in the heart of diabetic rats. These time-dependent changes were also confirmed in neonatal cardiomyocytes and H9c2 cells treated with HG. Moreover, the reduction rate of hyperoxidized Prxs was greatly improved in the HG 24h group, which had an elevated expression of Srx. Our data also show that HG-induced AP1 activation and Srx expression were almost abolished by JNK inhibitor and N-acetylcysteine (NAC). In addition, siRNA-Srx suppressed HG-induced ANP and β-MHC gene expression. These observations suggest that activation of AP1 induced by HG is important for the expression of Srx and the reduction of hyperoxidized Prxs in cardiomyocytes. This Srx induction maybe is the pivotal compensatory protection mechanism against oxidative stress in diabetes or hyperglycaemia. Most interestingly, hyperoxidized Prxs/Srx pathway may be involved in the cardiac hypertrophy signaling of diabetes.

Roles of p62 in BDNF-dependent autophagy suppression and neuroprotection against mitochondrial dysfunction in rat cortical neurons

Previously, we have reported that pre-conditioning of primary rat cortical neurons with brain-derived neurotrophic factor (BDNF) may exert neuroprotective effects against 3-nitropropionic acid (3-NP), a mitochondrial complex II inhibitor. However, the underlying mechanisms, especially potential involvements of autophagy, remain elusive. In this work, we tested the hypothesis that BDNF may suppress 3-NP-induced autophagy to exert its neuroprotective effects by inducing the expression of p62/sequestosome-1 in primary cortical neurons. We found that 3-NP increased total level of microtubule-associated protein 1A/1B-light chain (LC)-3 as well as the LC3-II/LC3-I ratio, an index of autophagy, in primary cortical neurons. BDNF decreased LC3-II/LC3-I ratio and time-dependently induced expression of p62. Knockdown of p62 by siRNA restored LC3-II/LC3-I ratio and increased total LC3 levels associated with BDNF exposure; p62 knockdown also abolished BDNF-dependent neuroprotection against 3-NP. Upstream of p62, we found that BDNF triggered phosphorylation of mammalian target of rapamycin (mTOR) and its downstream mediator p70S6K; importantly, the mTOR inhibitor rapamycin reduced both BDNF-dependent p62 induction as well as 3-NP resistance. BDNF is known to induce c-Jun in cortical neurons. We found that c-Jun knockdown in part attenuated BDNF-mediated p62 induction, whereas p62 knockdown had no significant effects on c-Jun expression. In addition to suppressing p62 induction, rapamycin also partially suppressed BDNF-induced c-Jun expression, but c-Jun knockdown failed to affect mTOR activation. Together, our results suggested that BDNF inhibits 3-NP-induced autophagy via, at least in part, mTOR/c-Jun-dependent induction of p62 expression, together contributing to neuroprotection against mitochondrial inhibition.

Sulfiredoxin Promotes Colorectal Cancer Cell Invasion and Metastasis through a Novel Mechanism of Enhancing EGFR Signaling

Sulfiredoxin (SRXN1/Srx) is a multifunction enzyme with a primary antioxidant role of reducing the overoxidized inactive form of peroxiredoxins (Prxs). The function and mechanisms of Srx in cancer development are not well understood. Here, Srx is preferentially expressed in human colorectal cancer cells but not in normal colon epithelial cells. Loss-of-function studies demonstrate that knockdown of Srx in poorly differentiated colorectal cancer cells not only leads to the inhibition of colony formation and cell invasion in vitro, but also reduces tumor xenograft growth and represses metastasis to distal organs in a mouse orthotopic implantation model. Notably, exactly opposite effects were observed in gain-of-function experiments when Srx was ectopically expressed in well-differentiated colorectal cancer cells. Mechanistically, expression of Srx enhances the activation of MAPK signaling through increasing the C-terminal tyrosine phosphorylation levels of EGFR. This function of Srx is mediated through its inhibition of EGFR acetylation at K1037, a novel posttranslational modification of EGFR in human colorectal cancer cells identified by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC/ESI/MS-MS) proteomic analysis. Furthermore, abolishment of K1037 acetylation in human colorectal cancer cells by site-specific mutagenesis leads to sustained activation of EGFR-MAPK signaling. Combined, these data reveal that Srx promotes colorectal cancer cell invasion and metastasis through a novel mechanism of enhancing EGFR signaling.

IMPLICATIONS

Sulfiredoxin is a critical oncogenic protein that can be used as a molecular target to develop therapeutics for patients with metastatic colorectal cancer.

BDNF Induces Striatal-Enriched Protein Tyrosine Phosphatase 61 Degradation Through the Proteasome

Brain-derived neurotrophic factor (BDNF) promotes synaptic strengthening through the regulation of kinase and phosphatase activity. Conversely, striatal-enriched protein tyrosine phosphatase (STEP) opposes synaptic strengthening through inactivation or internalization of signaling molecules. Here, we investigated whether BDNF regulates STEP levels/activity. BDNF induced a reduction of STEP61 levels in primary cortical neurons, an effect that was prevented by inhibition of tyrosine kinases, phospholipase C gamma, or the ubiquitin-proteasome system (UPS). The levels of pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204), two STEP substrates, increased in BDNF-treated cultures, and blockade of the UPS prevented STEP61 degradation and reduced BDNF-induced GluN2B and ERK1/2 phosphorylation. Moreover, brief or sustained cell depolarization reduced STEP61 levels in cortical neurons by different mechanisms. BDNF also promoted UPS-mediated STEP61 degradation in cultured striatal and hippocampal neurons. In contrast, nerve growth factor and neurotrophin-3 had no effect on STEP61 levels. Our results thus indicate that STEP61 degradation is an important event in BDNF-mediated effects.

Nuclear Factor-kappaB-Dependent Sestrin2 Induction Mediates the Antioxidant Effects of BDNF Against Mitochondrial Inhibition in Rat Cortical Neurons

Brain-derived neurotrophic factor (BDNF), in addition to its neurotrophic action, also possesses antioxidant activities. However, the underlying mechanisms remain to be fully defined. Sestrin2 is a stress-responsive gene implicated in the cellular defense against oxidative stress. Currently, the potential functions of sestrin2 in nervous system, in particular its correlation with neurotrophic factors, have not been well established. In this study, we hypothesized that BDNF may enhance sestrin2 expression to confer neuronal resistance against oxidative stress induced by 3-nitropropionic acid (3-NP), an irreversible mitochondrial complex II inhibitor, and characterized the molecular mechanisms underlying BDNF induction of sestrin2 in primary rat cortical cultures. We found that BDNF-mediated sestrin2 expression in cortical neurons required formation of nitric oxide (NO) with subsequent production of 3',5'-cyclic guanosine monophosphate (cGMP) and activation of cGMP-dependent protein kinase (PKG). BDNF induced localization of nuclear factor-kappaB (NF-κB) subunits p65 and p50 into neuronal nuclei that required PKG activities. Interestingly, BDNF exposure led to formation of a protein complex containing at least PKG-1 and p65/p50, which bound to sestrin2 promoter with resultant upregulation of its protein products. Finally, BDNF preconditioning mitigated production of reactive oxygen species (ROS) as a result of 3-NP exposure; this antioxidative effect of BDNF was dependent upon PKG activity, NF-κB, and sestrin2. Taken together, our results indicated that BDNF enhances sestrin2 expression to confer neuronal resistance against oxidative stress induced by 3-NP through attenuation of ROS formation; furthermore, BDNF induction of sestrin2 requires activation of a pathway involving NO/PKG/NF-κB.

Dimethyl Fumarate Protects Neural Stem/Progenitor Cells and Neurons from Oxidative Damage through Nrf2-ERK1/2 MAPK Pathway

Multiple sclerosis (MS) is the most common multifocal inflammatory demyelinating disease of the central nervous system (CNS). Due to the progressive neurodegenerative nature of MS, developing treatments that exhibit direct neuroprotective effects are needed. Tecfidera™ (BG-12) is an oral formulation of the fumaric acid esters (FAE), containing the active metabolite dimethyl fumarate (DMF). Although BG-12 showed remarkable efficacy in lowering relapse rates in clinical trials, its mechanism of action in MS is not yet well understood. In this study, we reported the potential neuroprotective effects of dimethyl fumarate (DMF) on mouse and rat neural stem/progenitor cells (NPCs) and neurons. We found that DMF increased the frequency of the multipotent neurospheres and the survival of NPCs following oxidative stress with hydrogen peroxide (H₂O₂) treatment. In addition, utilizing the reactive oxygen species (ROS) assay, we showed that DMF reduced ROS production induced by H₂O₂. DMF also decreased oxidative stress-induced apoptosis. Using motor neuron survival assay, DMF significantly promoted survival of motor neurons under oxidative stress. We further analyzed the expression of oxidative stress-induced genes in the NPC cultures and showed that DMF increased the expression of transcription factor nuclear factor-erythroid 2-related factor 2 (Nrf2) at both levels of RNA and protein. Furthermore, we demonstrated the involvement of Nrf2-ERK1/2 MAPK pathway in DMF-mediated neuroprotection. Finally, we utilized SuperArray gene screen technology to identify additional anti-oxidative stress genes (Gstp1, Sod2, Nqo1, Srxn1, Fth1). Our data suggests that analysis of anti-oxidative stress mechanisms may yield further insights into new targets for treatment of multiple sclerosis (MS).

Oncostatin M-dependent Mcl-1 induction mediated by JAK1/2-STAT1/3 and CREB contributes to bioenergetic improvements and protective effects against mitochondrial dysfunction in cortical neurons

Oncostatin M (OSM), a cytokine in the interleukin-6 (IL-6) family, has been proposed to play a protective role in the central nervous system, such as attenuation of excitotoxicity induced by N-methyl-D-aspartate (NMDA) and glutamate. However, the potential neuroprotective effects of OSM against mitochondrial dysfunction have never been reported. In the present study, we tested the hypothesis that OSM may confer neuronal resistance against 3-nitropropionic acid (3-NP), a plant toxin that irreversibly inhibits the complex II of the mitochondrial electron transport chain, and characterized the underlying molecular mechanisms. We found that OSM preconditioning dose- and time-dependently protected cortical neurons against 3-NP toxicity. OSM stimulated expression of myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic Bcl-2 family member expressed in differentiating myeloid cells, that required prior phosphorylation of Janus kinase-1 (JAK1), JAK2, extracellular signal-regulated kinase-1/2 (ERK1/2), signal transducer and activator of transcription-3 (STAT3), STAT1, and cAMP-response element-binding protein (CREB). Pharmacological inhibitors of JAK1, JAK2, ERK1/2, STAT3, STAT1, and CREB as well as the siRNA targeting at STAT3 and Mcl-1 all abolished OSM-dependent 3-NP resistance. Finally, OSM-dependent Mcl-1 induction contributed to the enhancements of mitochondrial bioenergetics including increases in spare respiratory capacity and ATP production. In conclusion, our findings indicated that OSM induces Mcl-1 expression via activation of ERK1/2, JAK1/2, STAT1/3, and CREB; furthermore, OSM-mediated Mcl-1 induction contributes to bioenergetic improvements and neuroprotective effects against 3-NP toxicity in cortical neurons. OSM may thus serve as a novel neuroprotective agent against mitochondrial dysfunction commonly associated with pathogenic mechanisms underlying neurodegeneration.

Inhibitor of Differentiation-1 and Hypoxia-Inducible Factor-1 Mediate Sonic Hedgehog Induction by Amyloid Beta-Peptide in Rat Cortical Neurons

One major pathological hallmark of Alzheimer's disease (AD) is the accumulation of senile plaques mainly composed of neurotoxic amyloid beta-peptide (Aβ) in the patients' brains. Sonic hedgehog (SHH) is a morphogen critically involved in the embryonic development of the central nervous system (CNS). In the present study, we tested whether Aβ may induce SHH expression and explored its underlying mechanisms. We found that both Aβ25-35 and Aβ1-42 enhanced SHH expression in the primary cortical neurons derived from fetal rat brains. Immunohistochemistry revealed heightened expression of SHH in the cortex and hippocampus of aged (9 and 12 months old) AD transgenic mouse brains as compared to age-matched littermate controls. Chromatin immunoprecipitation (ChIP) assay demonstrated that Aβ25-35 enhanced binding of hypoxia-inducible factor-1 (HIF-1) to the promoter of the Shh gene in primary cortical cultures; consistently, Aβ25-35 induction of SHH was abolished by HIF-1α small interfering RNA (siRNA). Aβ25-35 also time-dependently induced inhibitor of differentiation-1 (Id1) that has been shown to stabilize HIF-1α; further, Aβ25-35-mediated induction of HIF-1α and SHH was both suppressed by Id1 siRNA. Pharmacological induction of HIF-1α by cobalt chloride and application of the cell-permeable recombinant Id1 proteins were both sufficient to induce SHH expression. Finally, both the SHH pathway inhibitor cyclopamine and its neutralizing antibody attenuated Aβ cytotoxicity, albeit to a minor extent. These results thus established a signaling cascade of "Aβ → Id1 → HIF-1 → SHH" in primary rat cortical cultures; furthermore, SHH may in part contribute to Aβ neurotoxicity.

NAD attenuates oxidative DNA damages induced by amyloid beta-peptide in primary rat cortical neurons

One major pathological hallmark of Alzheimer's disease (AD) is accumulation of senile plaques in patients' brains, mainly composed of amyloid beta-peptide (Aβ). Nicotinamide adenine dinucleotide (NAD) has emerged as a common mediator regulating energy metabolism, mitochondrial function, aging, and cell death, all of which are critically involved in neuronal demise observed in AD. In this work, we tested the hypothesis that NAD may attenuate Aβ-induced DNA damages, thereby conferring neuronal resistance to primary rat cortical cultures. We found that co-incubation of NAD dose-dependently attenuated neurotoxicity mediated by Aβ25-35 and Aβ1-42 in cultured rat cortical neurons, with the optimal protective dosage at 50 mM. NAD also abolished the formation of reactive oxygen species (ROS) induced by Aβ25-35. Furthermore, Aβs were capable of inducing oxidative DNA damages by increasing the extents of 8-hydroxy-2´-deoxyguanosine (8-OH-dG), numbers of apurinic/apyrimidinic (AP) sites, genomic DNA single-stranded breaks (SSBs), as well as DNA double-stranded breaks (DSBs)/fragmentation, which can all be attenuated upon co-incubation with NAD. Our results thus reveal a novel finding that NAD is protective against DNA damage induced by existing Aβ, leading ultimately to neuroprotection in primary cortical culture.

Tumor promoter-induced sulfiredoxin is required for mouse skin tumorigenesis

Sulfiredoxin (Srx), the exclusive enzyme that reduces the hyperoxidized inactive form of peroxiredoxins (Prxs), has been found highly expressed in several types of human skin cancer. To determine whether Srx contributed to skin tumorigenesis in vivo, Srx null mice were generated on an FVB background. Mouse skin tumorigenesis was induced by a 7,12-dimethylbenz[α]anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA) protocol. We found that the number, volume and size of papillomas in Srx(-/-) mice were significantly fewer compared with either wild-type (Wt) or heterozygous (Het) siblings. Histopathological analysis revealed more apoptotic cells in tumors from Srx(-/-) mice. Mechanistic studies in cell culture revealed that Srx was stimulated by TPA in a redox-independent manner. This effect was mediated transcriptionally through the activation of mitogen-activated protein kinase and Jun-N-terminal kinase. We also demonstrated that Srx was capable of reducing hyperoxidized Prxs to facilitate cell survival under oxidative stress conditions. These findings suggested that loss of Srx protected mice, at least partially, from DMBA/TPA-induced skin tumorigenesis. Therefore, Srx has an oncogenic role in skin tumorigenesis and targeting Srx may provide novel strategies for skin cancer prevention or treatment.

Loss of sulfiredoxin renders mice resistant to azoxymethane/dextran sulfate sodium-induced colon carcinogenesis

Sulfiredoxin (Srx) is the enzyme that reduces the hyperoxidized inactive form of peroxiredoxins. To study the function of Srx in carcinogenesis in vivo, we tested whether loss of Srx protects mice from cancer development. Srx null mice were generated and colon carcinogenesis was induced by an azoxymethane (AOM) and dextran sulfate sodium (DSS) protocol. Compared with either wild-type (Wt) or heterozygotes, Srx(-/-) mice had significantly reduced rates in both tumor multiplicity and volume. Mechanistic studies reveal that loss of Srx did not alter tumor cell proliferation; however, increased apoptosis and decreased inflammatory cell infiltration were obvious in tumors from Srx null mice compared with those from Wt control. In addition to the AOM/DSS model, examination of Srx expression in human reveals a tissue-specific expression pattern. Srx expression was also demonstrated in tumors from colorectal cancer patients and the levels of expression were associated with patients' clinic stages. These data provide the first in vivo evidence that loss of Srx renders mice resistant to AOM/DSS-induced colon carcinogenesis, suggesting that Srx has a critical oncogenic role in cancer development, and Srx may be used as a marker for human colon cancer pathogenicity.

Effect of brain-derived neurotrophic factor on c-jun expression in the rd mouse retina

AIM

To determine the location of c-jun protein, dynamic changes in c-jun mRNA and protein expression, and ultrastructure characteristics in the rd mouse retina, following a single dose of brain-derived neurotrophic factor (BDNF) in a short period of time.

METHODS

A single intravitreal injection of BDNF at two dosages (25µg/L or 50µg/L) was given to the right eye of the rd mouse at age 2 and 3 weeks respectively. Two weeks after injection, the location of c-jun protein in the retina was observed by immunofluorescence detection, c-jun mRNA and protein expression in retinas were detected by quantitative real time polymerase chain reaction (RT-PCR) and western immunoblotting analysis, ultrastructure characteristics of retinas were detected by transmission electron microscope (TEM) observation.

RESULTS

c-jun protein was expressed in the inner nuclear layer (INL) of retina. BDNF at two dosages (25µg/L and 50µg/L) increased c-jun mRNA expression at PN-4 weeks respectively (P(1)=0.019, P(2)=0.021). 50µg/L BDNF increased c-jun protein expression at PN-4 weeks (P =0.000). The retinal ultrastructure was improved.

CONCLUSION

The effects of BDNF exerts on the c-jun expression in the retina are dose-dependent and time-dependent, which may mediate photoreceptor rescue indirectly in the pathological process of retinitis pigmentosa (RP) at early stage.