Prevention of endoplasmic reticulum stress-induced cell death by brain-derived neurotrophic factor in cultured cerebral cortical neurons

The Thr92Ala polymorphism in the type 2 deiodinase gene is linked to depression in patients with COVID-19 after hospital discharge

Background

The Thr92Ala-DIO2 polymorphism has been associated with clinical outcomes in hospitalized patients with COVID-19 and neuropsychiatric diseases. This study examines the impact of the Thr92Ala-DIO2 polymorphism on neuropsychological symptoms, particularly depressive symptoms, in patients who have had moderate to severe SARS-CoV-2 infection and were later discharged.

Methods

Our prospective cohort study, conducted from June to August 2020, collected data from 273 patients hospitalized with COVID-19. This included thyroid function tests, inflammatory markers, hematologic indices, and genotyping of the Thr92Ala-DIO2 polymorphism. Post-discharge, we followed up with 68 patients over 30 to 45 days, dividing them into depressive (29 patients) and non-depressive (39 patients) groups based on their Beck Depression Inventory scores.

Results

We categorized 68 patients into three groups based on their genotypes: Thr/Thr (22 patients), Thr/Ala (41 patients), and Ala/Ala (5 patients). Depressive symptoms were less frequent in the Thr/Ala group (29.3%) compared to the Thr/Thr (59.1%) and Ala/Ala (60%) groups (p = 0.048). The Thr/Ala heterozygous genotype correlated with a lower risk of post-COVID-19 depression, as shown by univariate and multivariate logistic regression analyses. These analyses, adjusted for various factors, indicated a 70% to 81% reduction in risk.

Conclusion

Our findings appear to be the first to show that heterozygosity for Thr92Ala-DIO2 in patients with COVID-19 may protect against post-COVID-19 depression symptoms up to 2 months after the illness.

Membrane fusion, potential threats, and natural antiviral drugs of pseudorabies virus

Pseudorabies virus (PrV) can infect several animals and causes severe economic losses in the swine industry. Recently, human encephalitis or endophthalmitis caused by PrV infection has been frequently reported in China. Thus, PrV can infect animals and is becoming a potential threat to human health. Although vaccines and drugs are the main strategies to prevent and treat PrV outbreaks, there is no specific drug, and the emergence of new PrV variants has reduced the effectiveness of classical vaccines. Therefore, it is challenging to eradicate PrV. In the present review, the membrane fusion process of PrV entering target cells, which is conducive to revealing new therapeutic and vaccine strategies for PrV, is presented and discussed. The current and potential PrV pathways of infection in humans are analyzed, and it is hypothesized that PrV may become a zoonotic agent. The efficacy of chemically synthesized drugs for treating PrV infections in animals and humans is unsatisfactory. In contrast, multiple extracts of traditional Chinese medicine (TCM) have shown anti-PRV activity, exerting its effects in different phases of the PrV life-cycle and suggesting that TCM compounds may have great potential against PrV. Overall, this review provides insights into developing effective anti-PrV drugs and emphasizes that human PrV infection should receive more attention.

The Role of the Signaling Pathways Involved in the Effects of Hydrogen Sulfide on Endoplasmic Reticulum Stress

Endoplasmic reticulum (ER) is a kind of organelle with multiple functions including protein synthesis, modification and folding, calcium storage, and lipid synthesis. Under stress conditions, ER homeostasis is disrupted, which is defined as ER stress (ERS). The accumulation of unfolded proteins in the ER triggers a stable signaling network named unfolded protein response (UPR). Hydrogen sulfide is an important signal molecule regulating various physiological and pathological processes. Recent studies have shown that H₂S plays an important role in many diseases by affecting ERS, but its mechanism, especially the signaling pathways, is not fully understood. Therefore, in this review, we summarize the recent studies about the signaling pathways involved in the effects of H₂S on ERS in diseases to provide theoretical reference for the related in-depth researches.

Dihydroartemisinin Ameliorates Decreased Neuroplasticity-Associated Proteins and Excessive Neuronal Apoptosis in APP/PS1 Mice

BACKGROUND

Alzheimer's disease (AD) is one of the worst neurodegenerative disorders worldwide, with extracellular senile plaques (SP), subsequent intracellular neurofibrillary tangles (NFTs) and final neuron loss and synaptic dysfunction as the main pathological characteristics. Excessive apoptosis is the main cause of irreversible neuron loss. Thus, therapeutic intervention for these pathological features has been considered a promising strategy to treat or prevent AD. Dihydroartemisin (DHA) is a widely used first-line drug for malaria. Our previous study showed that DHA treatment significantly accelerated Aβ clearance, improved memory and cognitive deficits in vivo and restored autophagic flux both in vivo and in vitro.

METHODS

The present study intended to explore the neuroprotective effect of DHA on neuron loss in APP/PS1 double-transgenic mice and the underlying mechanisms involved. Transmission electron microscope (TEM) analysis showed that DHA significantly reduced the swollen endoplasmic reticulum (ER) in APP/PS1 mice. Western blot analysis indicated that DHA upregulated the level of NeuN, NeuroD, MAP2, and synaptophysin and promoted neurite outgrowth. Meanwhile, DHA greatly corrected the abnormal levels of Brain-derived neurotrophic factor (BDNF) and rescued the neuronal loss in the hippocampal CA1 area. Western blot analysis revealed that DHA notably down-regulated the protein expression of full length caspase-3, cleaved caspase-3 and Bax. In parallel, the expression of the anti-apoptotic protein Bcl-2 increased after oral DHA treatment.

RESULTS

Altogether, these results indicate that DHA protected AD mice from neuron loss via promoting the expression of BDNF and other neuroplasticity-associated proteins and suppressing the inhibition of neuronal apoptosis.

Curcumin protects rat hippocampal neurons against pseudorabies virus by regulating the BDNF/TrkB pathway

Pseudorabies virus (PRV) infection can elicit nervous system disorders. Curcumin has been reported to have neuroprotective effects. However, whether curcumin can protect neurons against PRV infection and the underlying mechanisms remain unclear. In the present study, for the first time, the protective effects of curcumin against PRV-induced oxidative stress, apoptosis, and mitochondrial dysfunction in rat hippocampal neurons and the brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) pathway were investigated. Results indicated that PRV with a titer of 3.06 × 10⁶ TCID50 (50% tissue culture infective dose) induced oxidative damage of hippocampal neurons 2 h post-infection and that 10 μM curcumin improved the viability of PRV-infected hippocampal neurons. Blocking the BDNF/TrkB pathway reversed the neuroprotective effects of curcumin, which were imparted by decreasing the PRV-induced upregulation of nitric oxide synthase expression, repressing the PRV-activated mitochondrial apoptotic pathway, and mitochondrial dysfunction. To conclude, curcumin exhibited a neuroprotective role against PRV infection by upregulating the BDNF/TrkB pathway. This study provides insight into the anti-PRV neuroprotective application of curcumin and the underlying mechanism in the prophylaxis and treatment of neurological disorders caused by PRV infection.

Developing Insulin and BDNF Mimetics for Diabetes Therapy

Diabetes is a global public health concern nowadays. The majority of diabetes mellitus (DM) patients belong to type 2 diabetes mellitus (T2DM), which is highly associated with obesity. The general principle of current therapeutic strategies for patients with T2DM mainly focuses on restoring cellular insulin response by potentiating the insulin-induced signaling pathway. In late-stage T2DM, impaired insulin production requires the patients to receive insulin replacement therapy for maintaining their glucose homeostasis. T2DM patients also demonstrate a drop of brain-derived neurotrophic factor (BDNF) in their circulation, which suggests that replenishing BDNF or enhancing its downstream signaling pathway may be beneficial. Because of their protein nature, recombinant insulin or BDNF possess several limitations that hinder their clinical application in T2DM treatment. Thus, developing orally active "insulin pill" or "BDNF pill" is essential to provide a more convenient and effective therapy. This article reviews the current development of non-peptidyl chemicals that mimic insulin or BDNF and their potential as anti-diabetic agents.

Endoplasmic reticulum stress in the livers of BDNF heterozygous knockout mice

Context: Involvement of endoplasmic reticulum (ER) stress and brain-derived neurotrophic factor (BDNF) in hepatic lipid metabolism has been reported previously. Objective: The effects of chronic BDNF deficiency on ER stress response in the livers were examined in this study. Methods: BDNF^(+/-) mice, characterised by BDNF deficiency, and their wild-type (WT) littermates were used. The ER stress was induced by tunicamycin (Tm) (0.5 mg/kg, intraperitoneal). Animals were divided into four groups; WT, WT + Tm, BDNF^(+/-), and BDNF^(+/-)+Tm. Results: At the basal conditions, BDNF deficiency did not affect hepatic cell death or lipid accumulation. However, during ER stress, BDNF^(+/-)+Tm group showed increased apoptosis, GADD153 immunostaining, sterol regulatory element-binding protein-1c (SREBP-1c) level, and steatosis compared to the WT + Tm group. Conclusion: Endogenous BDNF might be protective against apoptosis through GADD153 suppression and steatosis via SREBP-1c suppression during ER stress. This effect of BDNF might be clinically important for type 2 diabetes and obesity, which are related with both ER stress and BDNF deficiency.

Cellular demolition: Proteins as molecular players of programmed cell death

Apoptosis, a well-characterized and regulated cell death programme in eukaryotes plays a fundamental role in developing or later-life periods to dispose of unwanted cells to maintain typical tissue architecture, homeostasis in a spatiotemporal manner. This silent cellular death occurs without affecting any neighboring cells/tissue and avoids triggering of immunological response. Furthermore, diminished forms of apoptosis result in cancer and autoimmune diseases, whereas unregulated apoptosis may also lead to the development of a myriad of neurodegenerative diseases. Unraveling the mechanistic events in depth will provide new insights into understanding physiological control of apoptosis, pathological consequences of abnormal apoptosis and development of novel therapeutics for diseases. Here we provide a brief overview of molecular players of programmed cell death with discussion on the role of caspases, modifications, ubiquitylation in apoptosis, removal of the apoptotic body and its relevance to diseases.

TrkB-Shc Signaling Protects against Hippocampal Injury Following Status Epilepticus

Temporal lobe epilepsy (TLE) is a common and commonly devastating form of human epilepsy for which only symptomatic therapy is available. One cause of TLE is an episode of de novo prolonged seizures [status epilepticus (SE)]. Understanding the molecular signaling mechanisms by which SE transforms a brain from normal to epileptic may reveal novel targets for preventive and disease-modifying therapies. SE-induced activation of the BDNF receptor tyrosine kinase, TrkB, is one signaling pathway by which SE induces TLE. Although activation of TrkB signaling promotes development of epilepsy in this context, it also reduces SE-induced neuronal death. This led us to hypothesize that distinct signaling pathways downstream of TrkB mediate the desirable (neuroprotective) and undesirable (epileptogenesis) consequences. We subsequently demonstrated that TrkB-mediated activation of phospholipase Cγ1 is required for epileptogenesis. Here we tested the hypothesis that the TrkB-Shc-Akt signaling pathway mediates the neuroprotective consequences of TrkB activation following SE. We studied measures of molecular signaling and cell death in a model of SE in mice of both sexes, including wild-type and TrkB^Shc/Shc mutant mice in which a point mutation (Y515F) of TrkB prevents the binding of Shc to activated TrkB kinase. Genetic disruption of TrkB-Shc signaling had no effect on severity of SE yet partially inhibited activation of the prosurvival adaptor protein Akt. Importantly, genetic disruption of TrkB-Shc signaling exacerbated hippocampal neuronal death induced by SE. We conclude that therapies targeting TrkB signaling for preventing epilepsy should spare TrkB-Shc-Akt signaling and thereby preserve the neuroprotective benefits.SIGNIFICANCE STATEMENT Temporal lobe epilepsy (TLE) is a common and devastating form of human epilepsy that lacks preventive therapies. Understanding the molecular signaling mechanisms underlying the development of TLE may identify novel therapeutic targets. BDNF signaling thru TrkB receptor tyrosine kinase is one molecular mechanism promoting TLE. We previously discovered that TrkB-mediated activation of phospholipase Cγ1 promotes epileptogenesis. Here we reveal that TrkB-mediated activation of Akt protects against hippocampal neuronal death in vivo following status epilepticus. These findings strengthen the evidence that desirable and undesirable consequences of status epilepticus-induced TrkB activation are mediated by distinct signaling pathways downstream of this receptor. These results provide a strong rationale for a novel therapeutic strategy selectively targeting individual signaling pathways downstream of TrkB for preventing epilepsy.

Transglutaminase 2 Induces Deficits in Social Behavior in Mice

Impairments in social behavior are highly implicated in many neuropsychiatric disorders. Recent studies indicate a role for endoplasmic reticulum (ER) stress in altering social behavior, but the underlying mechanism is not known. In the present study, we examined the role of transglutaminase 2 (TG2), a calcium-dependent enzyme known to be induced following ER stress, in social behavior in mice. ER stress induced by tunicamycin administration increased TG2 protein levels in the mouse prefrontal cortex (PFC). PFC-specific inhibition of TG2 attenuated ER stress-induced deficits in social behavior. Conversely, overexpression of TG2 in the PFC resulted in social behavior impairments in mice. In addition, systemic administration of cysteamine, a TG2 inhibitor, attenuated social behavior deficits. Our preliminary findings using postmortem human brain samples found increases in TG2 mRNA and protein levels in the middle frontal gyrus of subjects with autism spectrum disorder. These findings in mice and human postmortem brain samples identify changes in TG2 activity in the possible dysregulation of social behavior.

BDNF/TrkB Pathway Mediates the Antidepressant-Like Role of H2S in CUMS-Exposed Rats by Inhibition of Hippocampal ER Stress

Our previous works have shown that hydrogen sulfide (H₂S) significantly attenuates chronic unpredictable mild stress (CUMS)-induced depressive-like behaviors and hippocampal endoplasmic reticulum (ER) stress. Brain-derived neurotrophic factor (BDNF) generates an antidepressant-like effect by its receptor tyrosine protein kinase B (TrkB). We have previously found that H₂S upregulates the expressions of BDNF and p-TrkB in the hippocampus of CUMS-exposed rats. Therefore, the present work was to explore whether BDNF/TrkB pathway mediates the antidepressant-like role of H₂S by blocking hippocampal ER stress. We found that treatment with K252a (an inhibitor of BDNF/TrkB pathway) significantly increased the immobility time in the forced swim test and tail suspension test and increased the latency to feed in the novelty-suppressed feeding test in the rats cotreated with sodium hydrosulfide (NaHS, a donor of H₂S) and CUMS. Similarly, K252a reversed the protective effect of NaHS against CUMS-induced hippocampal ER stress, as evidenced by increases in the levels of ER stress-related proteins, glucose-regulated protein 78, CCAAT/enhancer binding protein homologous protein and cleaved caspase-12. Taken together, our results suggest that BDNF/TrkB pathway plays an important mediatory role in the antidepressant-like action of H₂S in CUMS-exposed rats, which is by suppression of hippocampal ER stress. These data provide a novel mechanism underlying the protection of H₂S against CUMS-induced depressive-like behaviors.

Brain-derived neurotrophic factor improves proliferation of endometrial epithelial cells by inhibition of endoplasmic reticulum stress during early pregnancy

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family binds to two transmembrane receptors; neurotrophic receptor tyrosine kinase 2 (NTRK2) with high affinity and p75 with low affinity. Although BDNF-NTRK2 signaling in the central nervous system is known, signaling in the female reproductive system is unknown. Therefore, we determined effects of BDNF on porcine endometrial luminal epithelial (pLE) cells isolated from Day 12 of pregnancy, as well as expression of BDNF and NTRK2 in endometria of cyclic and pregnant pigs. BDNF-NTRK2 genes were expressed in uterine glandular (GE) and luminal (LE) epithelia during early pregnancy. In addition, their expression in uterine GE and LE decreased with increasing parity of sows. Recombinant BDNF increased proliferation in pLE cells in a dose-dependent, as well as expression of PCNA and Cyclin D1 in nuclei of pLE cells. BDNF also activated phosphorylation of AKT, P70S6K, S6, ERK1/2, JNK, P38 proteins in pLE cells. In addition, cell death resulting from tunicamycin-induced ER stress was prevented when pLE cells were treated with the combination of tunicamycin and BDNF which also decreased cells in the Sub-G₁ phase of the cell cycle. Furthermore, tunicamycin-induced unfolded protein response genes were mostly down-regulated to the basal levels as compared to non-treated pLE cells. Our finding suggests that BDNF acts via NTRK2 to induce development of pLE cells for maintenance of implantation and pregnancy by activating cell signaling via the PI3K and MAPK pathways and by inhibiting ER stress.

Neuroprotective Effects of Protein Tyrosine Phosphatase 1B Inhibition against ER Stress-Induced Toxicity

Several lines of evidence suggest that endoplasmic reticulum (ER) stress plays a critical role in the pathogenesis of many neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Protein tyrosine phosphatase 1B (PTP1B) is known to regulate the ER stress signaling pathway, but its role in neuronal systems in terms of ER stress remains largely unknown. Here, we showed that rotenone-induced toxicity in human neuroblastoma cell lines and mouse primary cortical neurons was ameliorated by PTP1B inhibition. Moreover, the increase in the level of ER stress markers (eIF2α phosphorylation and PERK phosphorylation) induced by rotenone treatment was obviously suppressed by concomitant PTP1B inhibition. However, the rotenone-induced production of reactive oxygen species (ROS) was not affected by PTP1B inhibition, suggesting that the neuroprotective effect of the PTP1B inhibitor is not associated with ROS production. Moreover, we found that MG132-induced toxicity involving proteasome inhibition was also ameliorated by PTP1B inhibition in a human neuroblastoma cell line and mouse primary cortical neurons. Consistently, downregulation of the PTP1B homologue gene in Drosophila mitigated rotenone- and MG132-induced toxicity. Taken together, these findings indicate that PTP1B inhibition may represent a novel therapeutic approach for ER stress-mediated neurodegenerative diseases.

Hydrogen-rich saline attenuates hippocampus endoplasmic reticulum stress after cardiac arrest in rats

BACKGROUND

Hydrogen-rich saline can selectively scavenge reactive oxygen species (ROS) and protect brain against ischemia reperfusion (I/R) injury. Endoplasmic reticulum stress (ERS) has been implicated in the pathological process of cerebral ischemia. However, very little is known about the role of hydrogen-rich saline in mediating pathophysiological reactions to ERS after I/R injury caused by cardiac arrest.

METHODS

The rats were randomly divided into three groups, sham group (n=30), ischemia/reperfusion group (n=40) and hydrogen-rich saline group (n=40). The rats in experimental groups were subjected to 4min of cardiac arrest and followed by resuscitation. Then they were randomized to receive 5ml/kg of either hydrogen-rich saline or normal saline.

RESULTS

Hydrogen-rich saline significantly improves survival rate and neurological function. The beneficial effects of hydrogen-rich saline were associated with decreased levels of oxidative products, as well as the increased levels of antioxidant enzymes. Furthermore, the protective effects of hydrogen-rich saline were accompanied by the increased activity of glucose-regulated protein 78 (GRP78), the decreased activity of cysteinyl aspartate specific proteinase-12 (caspase-12) and C/EBP homologous protein (CHOP).

CONCLUSIONS

Hydrogen-rich saline attenuates brain I/R injury may through inhibiting hippocampus ERS after cardiac arrest in rats.

Experimental study of the protective effects of SYVN1 against diabetic retinopathy

Genetic factors play an important role in the pathogenesis of diabetic retinopathy (DR). While many studies have focused on genes that increase susceptibility to DR, herein, we aimed to explore genes that confer DR resistance. Previously, we identified Hmg CoA reductase degradation protein 1 (SYVN1) as a putative DR protective gene via gene expression analysis. Transgenic mice overexpressing SYVN1 and wild-type (WT) mice with streptozotocin-induced diabetes were used in this experiment. Retinal damage and vascular leakage were investigated 6 months after induction of diabetes by histopathological and retinal cell apoptosis analyses and by retinal perfusion of fluorescein isothiocyanate-conjugated dextran. Compared with diabetic WT mice, diabetic SYVN1 mice had significantly more cells and reduced apoptosis in the retinal ganglion layer. Retinal vascular leakage was significantly lower in diabetic SYVN1 mice than in diabetic WT mice. The expression levels of endoplasmic reticulum (ER) stress-related, pro-inflammatory, and pro-angiogenic genes were also analyzed. Lower expression levels were observed in diabetic SYVN1 mice than in WT controls, suggesting that SYVN1 may play an important role in inhibiting ER stress, chronic inflammation, and vascular overgrowth associated with DR. Thus, these results strongly supported our hypothesis that SYVN1 confers DR resistance.

A Comprehensive View of the Neurotoxicity Mechanisms of Cocaine and Ethanol

Substance use disorder is an emerging problem concerning to human health, causing severe side effects, including neurotoxicity. The use of illegal drugs and the misuse of prescription or over-the-counter drugs are growing in this century, being one of the major public health problems. Ethanol and cocaine are one of the most frequently used drugs and, according to the National Institute on Drug Abuse, their concurrent consumption is one of the major causes for emergency hospital room visits. These molecules act in the brain through different mechanisms, altering the nervous system function. Researchers have focused the attention not just in the mechanism of action of these drugs, but also in the mechanism by which they damage the nervous tissue (neurotoxicity). Therefore, the goal of the present review is to provide a global perspective about the mechanisms of the neurotoxicity of cocaine and ethanol.

Hydrogen sulfide inhibits homocysteine-induced endoplasmic reticulum stress and neuronal apoptosis in rat hippocampus via upregulation of the BDNF-TrkB pathway

AIM

Homocysteine (Hcy) can elicit neuronal cell death, and hyperhomocysteinemia is a strong independent risk factor for Alzheimer's disease. The aim of this study was to examine the effects of hydrogen sulfide (H2S) on Hcy-induced endoplasmic reticulum (ER) stress and neuronal apoptosis in rat hippocampus.

METHODS

Adult male SD rats were intracerebroventricularly (icv) injected with Hcy (0.6 μmol/d) for 7 d. Before Hcy injection, the rats were treated with NaHS (30 or 100 μmol·kg(-1)·d(-1), ip) and/or k252a (1 μg/d, icv) for 2 d. The apoptotic neurons were detected in hippocampal coronal slices with TUNEL staining. The expression of glucose regulated protein 78 (GRP78), C/EBP homologous protein (CHOP), cleaved caspase-12, and BDNF in the hippocampus were examined using Western blotting assays. The generation of H2S in the hippocampus was measured with the NNDPD method.

RESULTS

Hcy markedly inhibited the production of endogenous H2S and increased apoptotic neurons in the hippocampus. Furthermore, Hcy induced ER stress responses in the hippocampus, as indicated by the upregulation of GRP78, CHOP, and cleaved caspase-12. Treatment with the H2S donor NaHS increased the endogenous H2S production and BDNF expression in a dose-dependent manner, and significantly reduced Hcy-induced neuronal apoptosis and ER stress responses in the hippocampus. Treatment with k252a, a specific inhibitor of TrkB (the receptor of BDNF), abolished the protective effects of NaHS against Hcy-induced ER stress in the hippocampus.

CONCLUSION

H2S attenuates ER stress and neuronal apoptosis in the hippocampus of Hcy-treated rats via upregulating the BDNF-TrkB pathway.

Linking cocaine to endoplasmic reticulum in striatal neurons: role of glutamate receptors

The endoplasmic reticulum (ER) controls protein folding. Accumulation of unfolded and misfolded proteins in the ER triggers an ER stress response to accelerate normal protein folding or if failed to cause apoptosis. The ER stress response is a conserved cellular response in mammalian cells and is sensitive to various physiological or pathophysiological stimuli. Recent studies unravel that this response in striatal neurons is subject to the tight modulation by psychostimulants. Cocaine and amphetamines markedly increased expression of multiple ER stress reporter proteins in the dorsal striatum (caudate putamen) and other basal ganglia sites. This evoked ER stress response is mediated by activation of group I metabotropic glutamate receptors and N-methyl-D-aspartate receptors. Converging Ca(2+) signals derived from activation of these receptors activate the c-Jun N-terminal kinase pathway to evoke ER stress responses. The discovery of robust ER stress responses to stimulant exposure establishes a previously unrecognized stimulant-ER coupling. This inducible coupling seems to contribute to neurotoxicity of stimulants related to various neuropsychiatric and neurodegenerative illnesses. Elucidating cellular mechanisms linking cocaine and other stimulants to ER is therefore important for the development of therapeutic agents for treating neurological disorders resulted from stimulant toxicity.

Analysis of the role of nerve growth factor in promoting cell survival during endoplasmic reticulum stress in PC12 cells

Nerve growth factor (NGF) was first described by Rita Levi-Montalcini in the early 1960s from her studies of peripheral neurons. It has since been reported that NGF has the potential to elongate neurites or to prevent apoptosis via specific intracellular mechanisms. It has further been reported that as a component of these mechanisms, NGF binds to a specific receptor, TrkA, and thereby contributes to peripheral nerve cell functions or neuronal functions. It is noteworthy in this regard that pheochromocytoma 12 (PC12) cells express TrkA and respond to neurite outgrowth or anti-apoptotic signals by binding to NGF. Hence, PC12 cells have been used as an in vitro model system for the study of neuronal functions. It has been reported that endoplasmic reticulum (ER) stress is involved in neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease. The common link with regard to ER stress is that the neuronal cells die in these pathologies via specific intracellular mechanisms. This type of cell death, if it is apoptotic in nature, is termed ER stress-mediated apoptosis. In the process of ER stress-mediated apoptosis, the cleavage of pro-caspase-12 residing on the ER and the expression of glucose-regulated protein 78 (GRP78) can be observed. The expression of GRP78 protein is a characteristic of an unfolded protein response (UPR) via specific signal transduction pathways mediated by the unfolded protein response element (UPRE) in the upstream region of the grp78 gene so on. In ER stress-mediated apoptosis, a caspase cascade is also observed. To further clarify the mechanisms underlying ER stress-mediated apoptosis, a better understanding of the UPR is therefore important. In our current study, we describe a method for detecting gene induction via the UPR, focusing on GRP78 and caspase activities as the measurement end-points. The information generated by our method will accelerate our understanding of the pathophysiological processes leading to ER stress-mediated apoptosis.

Ischemic postconditioning protects brain from ischemia/reperfusion injury by attenuating endoplasmic reticulum stress-induced apoptosis through PI3K-Akt pathway

Endoplasmic reticulum (ER) stress has been implicated in the pathology of cerebral ischemia. During prolonged period of stress or when the adaptive response fails, apoptotic cell death ensues. Cerebral ischemic postconditioning (Postcond) has been shown to reduce cerebral ischemia/reperfusion (I/R) injury in both focal and global cerebral ischemia model. However, the mechanism remains to be understood. This study aimed to elucidate whether Postcond attenuates brain I/R damage by suppressing ER stress-induced apoptosis and if the phosphatidylinositol-3kinase/Akt (PI3K/Akt) pathway is involved. A focal cerebral ischemia rat model was used in the study. Rat brain infarct size and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positive cells in ischemic penumbra were assessed after reperfusion of the brain. The expressions of C/EBP-homologous protein (CHOP), caspase-12, glucose-regulated protein 78 (GRP78) and the phosphorylation of Akt (Ser473) in ischemic penumbra were measured after reperfusion. Our results showed that Postcond significantly attenuated brain I/R injury, as shown by reduction in infarct size, cell apoptosis, CHOP expression, caspase-12 activation and increase in GRP78 expression. LY294002, a phosphoinositide 3-kinase inhibitor, increased the number of TUNEL-positive cells suppressed by Postcond in penumbra. In addition, LY294002 diminished the effect of Postcond on the activation of CHOP, caspase-12 and GRP78. These results suggest that Postcond protects brain from I/R injury by suppressing ER stress-induced apoptosis and PI3K/Akt pathway is involved.

Tunicamycin-induced cell death in the trigeminal ganglion is suppressed by nerve growth factor in the mouse embryo

The effect of nerve growth factor (NGF) on tunicamycin (Tm)-treated neurons in the trigeminal ganglion was investigated by use of caspase-3 immunohistochemistry. In intact embryos at embryonic day 16.5, only a few caspase-3-immunoreactivity were detected in the ganglion neurons. Mean +/- SE of the density of the immunoreactivity was 0.22 +/- 0.03%. In contrast, the number of the immunoreactive neurons was increased at 24 h after injection of 0.5 microg Tm in 1 microl of 0.05 N NaOH solution into mouse embryos at embryonic day 15.5. The density of immunoreactivity was also increased (mean +/- SE = 1.44 +/- 0.11%) compared to intact and 0.05 N NaOH-treated embryos (mean +/- SE = 0.35 +/- 0.03%). The Tm treatment caused increase of the number of trigeminal neurons representing apoptotic profiles (intact, mean +/- SE = 79.3 +/- 8.5; 0.05 N NaOH, mean +/- SE = 132 +/- 11.5; 0.5 microg Tm, mean +/- SE = 370.2 +/- 64.8). In addition, NGF significantly prevented the increase of density of the immunoreactivity (mean +/- SE = 0.54 +/- 0.16%) and the number of apoptotic cells (mean +/- SE = 146.2 +/- 11.3). Saline application (without NGF) had no effect on Tm-induced increase of the immunoreactivity (mean +/- SE = 1.78 +/- 0.23%) or the apoptotic profiles (mean +/- SE = 431.9 +/- 80.5). These results indicate that Tm-induced cell death in the trigeminal ganglion is suppressed by NGF in the mouse embryo.

Nerve growth factor attenuates 2-deoxy-d-glucose-triggered endoplasmic reticulum stress-mediated apoptosis via enhanced expression of GRP78

The glucose analog 2-deoxy-d-glucose (2DG) depletes cells of glucose. Inhibition of glycosylation caused by glucose depletion induces endoplasmic reticulum (ER) stress with subsequent apoptosis. Glucose-regulated protein 78 (GRP78) is a molecular chaperone that acts within the ER. During ER stress, GRP78 expression is induced as part of the unfolded protein response (UPR). We found that nerve growth factor (NGF) prevented 2DG-triggered ER stress-mediated apoptosis, but not the induction of GRP78 expression, in PC12 cells. Surprisingly, GRP78 expression was further up-regulated when NGF was added to 2DG-treated PC12 cells. When a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K), LY294002, was added to 2DG plus NGF-treated cells, both the effects of NGF on 2DG-induced apoptosis and GRP78 expression were significantly diminished. In addition, versipelostatin (VST), a specific inhibitor of GRP78 expression, and small interfering RNA (siRNA) against GRP78 mRNA also decreased both the effects of NGF on 2DG-induced apoptosis and GRP78 expression. RT-PCR and Western blot analyses revealed that enhanced production of nuclear p50 ATF6, but not spliced XBP1, mainly contributed to the NGF-induced enhancement of GRP78 expression in 2DG-treated cells. These results suggest that the NGF-activated PI3-K/Akt signaling pathway plays a protective role against ER stress-mediated apoptosis via enhanced expression of GRP78 in PC12 cells.

Inhibition of apoptosis signal-regulating kinase 1 reduces endoplasmic reticulum stress and nuclear huntingtin fragments in a mouse model of Huntington disease

Huntington's disease (HD) is characterized clinically by chorea, psychiatric disturbances, and dementia, while it is characterized pathologically by neuronal inclusions as well as striatal and cortical neurodegeneration. The neurodegeneration arises from the loss of long projection neurons in the cortex and striatum. In this study, we investigated the role of apoptosis signal-regulating kinase 1 (Ask1) in the pathogenesis of HD. We analyzed the expression of Ask1 and huntingtin (htt) within the striatum and cortex and also examined the interaction of Ask1 with htt fragments in HD (R6/2) mice. Additionally, we inhibited Ask1 and analyzed the resulting changes in brain-derived neurotrophic factor (BDNF) expression, motor function, and striatal atrophy. Ask1 activity was blocked using an Ask1 antibody raised against the C-terminus of the Ask1 protein. The anti-Ask1 antibody was infused into the striatum of the HD mice for four weeks using a micro-osmotic pump. The levels of Ask1 protein and endoplasmic reticulum (ER) stress were increased in HD mice. Binding of inactivated Ask1 to htt fragments was more prevalent in the cytosol than the nucleus of cortical neurons. Binding of inactivated Ask1 to htt fragments prevented translocation of the htt fragments into the nucleus, resulting in an improvement in motor dysfunction and atrophy. In the normal state, active Ask1 may help htt fragments enter the nucleus, while inactivated Ask1 hinders this translocation by binding to but not releasing fragmented htt into the nucleus. We propose that Ask1 may interact with htt fragments and subsequently induce ER stress. BDNF depletion may be prevented by targeting Ask1; this would decrease ER stress and possibly ameliorate behavioral or anatomical abnormalities that accompany HD. Therefore, regulating the amounts and activity of the Ask1 protein is a novel strategy for treatment of HD.

Endoplasmic reticulum stress and diabetic retinopathy

Endoplasmic reticulum (ER) stress is involved in the pathogenesis of several diseases including Alzheimer disease and Parkinson disease. Many recent studies have shown that ER stress is related to the pathogenesis of diabetes mellitus, and with the death of pancreatic beta-cells, insulin resistance, and the death of the vascular cells in the retina. Diabetic retinopathy is a major complication of diabetes and results in death of both neural and vascular cells. Because the death of the neurons directly affects visual function, the precise mechanism causing the death of neurons in early diabetic retinopathy must be determined. The ideal therapy for preventing the onset and the progression of diabetic retinopathy would be to treat the factors involved with both the vascular and neuronal abnormalities in diabetic retinopathy. In this review, we present evidence that ER stress is involved in the death of both retinal neurons and vascular cells in diabetic eyes, and thus reducing or blocking ER stress may be a potential therapy for preventing the onset and the progression of diabetic retinopathy.

Endoplasmic reticulum stress and diabetic retinopathy

Endoplasmic reticulum (ER) stress is involved in the pathogenesis of several diseases including Alzheimer disease and Parkinson disease. Many recent studies have shown that ER stress is related to the pathogenesis of diabetes mellitus, and with the death of pancreatic β-cells, insulin resistance, and the death of the vascular cells in the retina. Diabetic retinopathy is a major complication of diabetes and results in death of both neural and vascular cells. Because the death of the neurons directly affects visual function, the precise mechanism causing the death of neurons in early diabetic retinopathy must be determined. The ideal therapy for preventing the onset and the progression of diabetic retinopathy would be to treat the factors involved with both the vascular and neuronal abnormalities in diabetic retinopathy. In this review, we present evidence that ER stress is involved in the death of both retinal neurons and vascular cells in diabetic eyes, and thus reducing or blocking ER stress may be a potential therapy for preventing the onset and the progression of diabetic retinopathy.

The protection of Bcl-2 overexpression on rat cortical neuronal injury caused by analogous ischemia/reperfusion in vitro

Recent studies have suggested that neuronal apoptosis in cerebral ischemia could arise from dysfunction of endoplasmic reticulum (ER) and mitochondria. B-cell lymphoma/leukemia-2 gene (Bcl-2) has been described as an inhibitor both in programmed cell death (PCD) and ER dysfunction during apoptosis, and the Bcl-2 family play a key role in regulating the PCD, both locally at the ER and from a distance at the mitochondrial membrane. However, its signal pathways and concrete mechanisms in endoplasmic reticulum-initiated apoptosis remain incompletely understood. We therefore investigate whether ischemia/reperfusion (I/R) causes neuronal apoptosis in part via cross-talk between ER and mitochondria or not, and how the overexpression of Bcl-2 prevents this form of cell death. Here we show that analogous I/R-induced cell death occurs consequent to interactions of ER stress and mitochondrial death pathways. The participation of the mitochondrial pathway was demonstrated by the release of cytochrome C (cyt C) from mitochondrial into cytoplasmic fractions and caspase-9 cleavage. The involvement of ER stress was further supported by the observable increase of glucose-regulated protein 78(GRP78)/BiP expression and caspase-12 activity. Furthermore, prior to these changes, swelling of the ER lumen and dissociation of ribosomes from rough ER were detected by electron microscopy. Bcl-2 overexpression inhibits the release of cyt C and the activation of caspase-9/-8/-3 but not caspase-12 based on the results of Western blot. These suggest that cross-talk between ER and mitochondria participate in neuronal damage after ischemia/reperfusion. Bcl-2 overexpression could suppress I/R-induced neuronal apoptosis via influencing mitochondrial integrity.

Nerve growth factor blocks thapsigargin-induced apoptosis at the level of the mitochondrion via regulation of Bim

This study examined how the neurotrophin, nerve growth factor (NGF), protects PC12 cells against endoplasmic reticulum (ER) stress-induced apoptosis. ER stress was induced using thapsigargin (TG) that inhibits the sarcoplasmic/ER Ca2+-ATPase pump (SERCA) and depletes ER Ca2+ stores. NGF pre-treatment inhibited translocation of Bax to the mitochondria, loss of mitochondrial transmembrane potential, cytochrome c release, activation of caspases (-3, -7 and -9) and apoptosis induction by TG. Notably, TG also caused a marked induction of BimEL mRNA and protein, and knockdown of Bim with siRNA protected cells against TG-induced apoptosis. NGF delayed the induction and increased the phosphorylation of BimEL. NGF-mediated protection was dependent on phosphatidylinositol-3 kinase (PI3K) signalling since all above apoptotic events, including expression and phosphorylation status of BimEL protein, could be reverted by the PI3K inhibitor LY294002. In contrast, NGF had no effect on the TG-mediated induction of the unfolded protein response (increased expression of Grp78, GADD34, splicing of XBP1 mRNA) or ER stress-associated pro-apoptotic responses (induction of C/EBP homologous protein [CHOP], induction and processing of caspase-12). These data indicate that NGF-mediated protection against ER stress-induced apoptosis occurs at the level of the mitochondria by regulating induction and activation of Bim and mitochondrial translocation of Bax.

Cocaine increases immunoglobulin heavy chain binding protein and caspase-12 expression in the rat dorsal striatum

RATIONALE

Cocaine increases endoplasmic reticulum (ER) stress protein expression via glutamate and dopamine receptor activation in the dorsal striatum.

OBJECTIVES

The present study was performed to investigate ER stress response in the dorsal striatum in response to acute or repeated cocaine stimulation. It was hypothesized that cocaine upregulates the ER stress protein immunoglobulin heavy chain binding protein (BiP) and the ER stress-associated protein caspase-12 via N-methyl-D-aspartate (NMDA) and D1 dopamine receptor activation.

MATERIALS AND METHODS

Western immunoblot and immunohistochemical analyses were mainly performed to test this hypothesis in the rat dorsal striatum.

RESULTS

The results showed that BiP and caspase-12 immunoreactivities were significantly increased at 30, 60, and 120 min after acute or repeated intraperitoneal (i.p.) injections of three doses (10, 20, 40 mg/kg) of cocaine for seven consecutive days. Intrastriatal (i.s.) infusion of the selective NMDA antagonist MK801 (2 nmol) or AP5 (2 nmol) significantly attenuated the increase in the immunoreactivity of caspase-12 in the dorsal striatum induced by repeated, but not acute, cocaine (20 mg/kg) administration. However, i.p. injection of the selective D1 antagonist SCH23390 (0.1 mg/kg) significantly attenuated the increase in the immunoreactivity of caspase-12 in the dorsal striatum induced by both acute and repeated cocaine (20 mg/kg) stimulation.

CONCLUSION

These findings suggest that acute or repeated cocaine administration can cause ER stress response in the dorsal striatum in which NMDA and D1 dopamine receptors participate in the mediation of the process.

Brain-derived neurotrophic factor suppresses tunicamycin-induced upregulation of CHOP in neurons

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) lumen triggers ER stress. ER stress initiates a number of specific compensatory signaling pathways including unfolded protein response (UPR). UPR is characterized by translational attenuation, synthesis of ER chaperone proteins such as glucose-regulated protein of 78 kDa (GRP78, also known as Bip), and transcriptional induction, which includes the activation of transcription factors such as activating transcriptional factor 6 (ATF6) and C/EBP homologous protein (CHOP, also known as growth arrest and DNA damage-inducible gene 153 [GADD153]). Sustained ER stress ultimately leads to cell death. ER functions are believed to be impaired in various neurodegenerative diseases, as well as in some acute disorders of the brain. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, functions as a neuroprotective agent and rescues neurons from various insults. The molecular mechanisms underlying BDNF neuroprotection, however, remain to be elucidated. We showed that CHOP partially mediated ER stress-induced neuronal death. BDNF suppressed ER stress-induced upregulation/ nuclear translocation of CHOP. The transcription of CHOP is regulated by ATF4, ATF6, and XBP1; BDNF selectively blocked the ATF6/CHOP pathway. Furthermore, BDNF inhibited the induction of death receptor 5 (DR5), a transcriptional target of CHOP. Our study thus suggests that suppression of CHOP activation may contribute to BDNF-mediated neuroprotection during ER stress responses.

Unfolded Protein Response after Neurotrauma

The endoplasmic reticulum (ER) lumen, which actively monitors the synthesis, folding, and modification of newly synthesized transmembrane and secretory proteins as well as lipids, is quite sensitive to homeostatic perturbations. The biochemical, molecular, and physiological events that elevate cellular ER stress levels and disrupt Ca2+ homeostasis trigger secondary reactions. These reactions are factors in the ongoing neurological pathology contributing to the continual tissue loss. However, the cells are not without defensive systems. One of the reactive mechanisms, the unfolded protein response (UPR), when evoked, provides some measure of protection, unless the stress conditions become prolonged or overwhelming. UPR activation occurs when key ER membrane-bound sensor proteins detect the excess accumulation of misfolded or unfolded proteins within the ER lumen. The activation of these sensors leads to a general protein translation shut-down, transcriptional induction, and translation of select proteins to deal with the difficult and miscreant protein or to encourage their degradation so they will do no harm. If the stress is prolonged, caspase-12, along with other apoptotic proteins, are activated, triggering programmed cell death. UPR, once considered to be a rather simple response, can now be characterized as a multifaceted labyrinth of reactions that continues expanding as research intensifies. This review will examine what has been learned to date about how this highly efficient and specific signaling pathway copes with ER stress, by centering on the basic components, their roles, and the complex interactions engendered. Finally, the UPR impact in various central nervous system injuries is summarized.

ER stress and neurodegenerative diseases

Endoplasmic reticulum (ER) stress is caused by disturbances in the structure and function of the ER with the accumulation of misfolded proteins and alterations in the calcium homeostasis. The ER response is characterized by changes in specific proteins, causing translational attenuation, induction of ER chaperones and degradation of misfolded proteins. In case of prolonged or aggravated ER stress, cellular signals leading to cell death are activated. ER stress has been suggested to be involved in some human neuronal diseases, such as Parkinson's disease, Alzheimer's and prion disease, as well as other disorders. The exact contributions to and casual effects of ER stress in the various disease processes, however, are not known. Here we will discuss the possible role of ER stress in neurodegenerative diseases, and highlight current knowledge in this field that may reveal novel insight into disease mechanisms and help to design better therapies for these disorders.

NGF-induced phosphatidylinositol 3-kinase signaling pathway prevents thapsigargin-triggered ER stress-mediated apoptosis in PC12 cells

Tunicamycin, an inhibitor of the glycosylation of newly biosynthesized proteins, induces endoplasmic reticulum (ER) stress and subsequent apoptosis, and caspase family proteases are activated during the process of ER stress-mediated apoptosis. In the present study, we showed that thapsigargin (Th), an inhibitor of the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA), also induced ER stress-mediated apoptosis, and nerve growth factor (NGF) prevented the apoptosis in PC12 cells. We also found that LY 294002, an inhibitor of phosphatidylinositol 3-kinase (PI 3-K), reduced the survival of cells treated with NGF for 24h in the presence of Th. We discovered that the activities of caspase-3, -9 and -12 were increased time-dependently after the treatment with Th, and NGF suppressed the Th-triggered activation of caspase-3, -9 and -12. LY 294002 diminished the effect of NGF on the inactivation of all these caspases. These results indicate that the NGF-induced PI 3-K signaling pathway prevents Th-triggered ER stress-specific apoptosis via inhibition of caspase-mediated apoptotic signal.

Dilinoleoyl-phosphatidylethanolamine from Hericium erinaceum protects against ER stress-dependent Neuro2a cell death via protein kinase C pathway

In many types of neurodegeneration, neuronal cell death is induced by endoplasmic reticulum (ER) stress. Hence, natural products able to reduce ER stress are candidates for use in the attenuation of neuronal cell death and, hence, in the reduction of the damage, which occurs in neurodegenerative disease. In this study, we investigated ER stress-reducing natural products from an edible mushroom, Hericium erinaceum. As a result of screening by cell viability assay on the protein glycosylation inhibitor tunicamycin-induced (i.e., ER stress-dependent) cell death, we found that dilinoleoyl-phosphatidylethanolamine (DLPE) was one of the molecules effective at reducing ER stress-dependent cell death in the mouse neuroblastoma cell line Neuro2a cells. A purified DLPE, commercially available, also exhibited a reducing effect on this ER stress-dependent cell death. Therefore, we concluded that DLPE has potential as a protective molecule in ER stress-induced cell death. From the structure of DLPE, it was hypothesized that it might activate protein kinase C (PKC). The activity of PKC-epsilon, a novel-type PKC, was increased by adding DLPE, and PKC-gamma, a conventional-type PKC, was activated on the coaddition of diolein and DLPE, as shown by in vitro enzyme activity analysis. The protecting activity of DLPE was attenuated in the presence of a PKC inhibitor GF109203X but not completely diminished. Therefore, DLPE can protect neuronal cells from ER stress-induced cell death, at least in part by the PKC pathway.