The integrated stress response pathway and neuromodulator signaling in the brain: lessons learned from dystonia

The integrated stress response (ISR) is a highly conserved biochemical pathway involved in maintaining proteostasis and cell health in the face of diverse stressors. In this Review, we discuss a relatively noncanonical role for the ISR in neuromodulatory neurons and its implications for synaptic plasticity, learning, and memory. Beyond its roles in stress response, the ISR has been extensively studied in the brain, where it potently influences learning and memory, and in the process of synaptic plasticity, which is a substrate for adaptive behavior. Recent findings demonstrate that some neuromodulatory neuron types engage the ISR in an "always-on" mode, rather than the more canonical "on-demand" response to transient perturbations. Atypical demand for the ISR in neuromodulatory neurons introduces an additional mechanism to consider when investigating ISR effects on synaptic plasticity, learning, and memory. This basic science discovery emerged from a consideration of how the ISR might be contributing to human disease. To highlight how, in scientific discovery, the route from starting point to outcomes can often be circuitous and full of surprise, we begin by describing our group's initial introduction to the ISR, which arose from a desire to understand causes for a rare movement disorder, dystonia. Ultimately, the unexpected connection led to a deeper understanding of its fundamental role in the biology of neuromodulatory neurons, learning, and memory.

Temporal regulation of transgene expression controlled by amino acid availability in human T cells

T cell therapy strategies, from allogeneic stem cell transplantation toward genetically-modified T cells infusion, develop powerful anti-tumor effects but are often accompanied by side effects and their efficacy remains sometimes to be improved. It therefore appears important to provide a flexible and easily reversible gene expression regulation system to control T cells activity. We developed a gene expression regulation technology that exploits the physiological GCN2-ATF4 pathway's ability to induce gene expression in T cells in response to one essential amino acid deficiency. We first demonstrated the functionality of NUTRIREG in human T cells by transient expression of reporter genes. We then validated that NUTRIREG can be used in human T cells to transiently express a therapeutic gene such as IL-10. Overall, our results represent a solid basis for the promising use of NUTRIREG to regulate transgene expression in human T cells in a reversible way, and more generally for numerous preventive or curative therapeutic possibilities in cellular immunotherapy strategies.

Strategies targeting endoplasmic reticulum stress to improve Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder with motor symptoms, which is caused by the progressive death of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). Accumulating evidence shows that endoplasmic reticulum (ER) stress occurring in the SNpc DA neurons is an early event in the development of PD. ER stress triggers the activation of unfolded protein response (UPR) to reduce stress and restore ER function. However, excessive and continuous ER stress and UPR exacerbate the risk of DA neuron death through crosstalk with other PD events. Thus, ER stress is considered a promising therapeutic target for the treatment of PD. Various strategies targeting ER stress through the modulation of UPR signaling, the increase of ER's protein folding ability, and the enhancement of protein degradation are developed to alleviate neuronal death in PD models. In this review, we summarize the pathological role of ER stress in PD and update the strategies targeting ER stress to improve ER protein homeostasis and PD-related events.

Molecular Mechanism Underlying Role of the XBP1s in Cardiovascular Diseases

Spliced X-box binding protein-1 (XBP1s) is a protein that belongs to the cAMP-response element-binding (CREB)/activating transcription factor (ATF) b-ZIP family with a basic-region leucine zipper (bZIP). There is mounting evidence to suggest that XBP1s performs a critical function in a range of different cardiovascular diseases (CVDs), indicating that it is necessary to gain a comprehensive knowledge of the processes involved in XBP1s in various disorders to make progress in research and clinical therapy. In this research, we provide a summary of the functions that XBP1s performs in the onset and advancement of CVDs such as atherosclerosis, hypertension, cardiac hypertrophy, and heart failure. Furthermore, we discuss XBP1s as a novel therapeutic target for CVDs.

Activation of the eIF2α-ATF4 Pathway by Chronic Paracetamol Treatment Is Prevented by Dietary Supplementation with Cysteine

Chronic treatment with acetaminophen (APAP) induces cysteine (Cys) and glutathione (GSH) deficiency which leads to adverse metabolic effects including muscle atrophy. Mammalian cells respond to essential amino acid deprivation through the phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). Phosphorylated eIF2α leads to the recruitment of activating transcription factor 4 (ATF4) to specific CCAAT/enhancer-binding protein-ATF response element (CARE) located in the promoters of target genes. Our purpose was to study the activation of the eIF2α-ATF4 pathway in response to APAP-induced Cys deficiency, as well as the potential contribution of the eIF2α kinase GCN2 and the effect of dietary supplementation with Cys. Our results showed that chronic treatment with APAP activated both GCN2 and PERK eIF2α kinases and downstream target genes in the liver. Activation of the eIF2α-ATF4 pathway in skeletal muscle was accompanied by muscle atrophy even in the absence of GCN2. The dietary supplementation with cysteine reversed APAP-induced decreases in plasma-free Cys, liver GSH, muscle mass, and muscle GSH. Our new findings demonstrate that dietary Cys supplementation also reversed the APAP-induced activation of GCN2 and PERK and downstream ATF4-target genes in the liver.

An Overview of Methods for Detecting eIF2α Phosphorylation and the Integrated Stress Response

Phosphorylation of the translation initiation factor eIF2α is an adaptive signaling event that is essential for cell and organismal survival from yeast to humans. It is central to the integrated stress response (ISR) that maintains cellular homeostasis in the face of threats ranging from viral infection, amino acid, oxygen, and heme deprivation to the accumulation of misfolded proteins in the endoplasmic reticulum. Phosphorylation of eIF2α has broad physiological, pathological, and therapeutic relevance. However, despite more than two decades of research and growing pharmacological interest, phosphorylation of eIF2α remains difficult to detect and quantify, because of its transient nature and because substoichiometric amounts of this modification are sufficient to profoundly reshape cellular physiology. This review aims to provide a roadmap for facilitating a robust evaluation of eIF2α phosphorylation and its downstream consequences in cells and organisms.

Endoplasmic reticulum stress and autophagy are involved in adipocyte-induced fibrosis in hepatic stellate cells

Liver fibrosis, with the characterization of progressive accumulation of extracellular matrix (ECM), is the common pathologic feature in the process of chronic liver disease. Hepatic stellate cells (HSCs) which are activated and differentiate into proliferative and contractile myofibroblasts are recognized as the main drivers of fibrosis. Obesity-related adipocytokine dysregulation is known to accelerate liver fibrosis progression, but the direct fibrogenic effect of mature adipocytes on HSCs has been rarely reported. Therefore, the purpose of this study was to explore the fibrogenic effect of adipocyte 3T3-L1 cells on hepatic stellate LX-2 cells. The results showed that incubating LX-2 cells with the supernatant of 3T3-L1 adipocytes triggered the expression of ECM related proteins, such as α-smooth muscle actin (α-SMA), type I collagen (CO-I), and activated TGF β/Smad2/3 signaling pathway in LX-2 cells. In addition, 3T3-L1 cells inhibited insulin sensitivity, activated endoplasmic reticulum stress and autophagy to promote the development of fibrosis. These results supported the notion that mature adipocytes can directly activate hepatic stellate cells, and the establishment of an in vitro model of adipocytes on HSCs provides an insight into screening of drugs for liver diseases, such as nonalcoholic fatty liver disease.

Isocaloric low protein diet in a mouse model for vanishing white matter does not impact ISR deregulation in brain, but reveals ISR deregulation in liver

Objective: Vanishing white matter (VWM) is a genetic brain white matter disorder caused by mutations in eIF2B. eIF2B is central in the integrated stress response (ISR), during which its activity is inhibited by various cellular stresses. VWM is a chronic progressive disease with episodes of rapid neurological deterioration provoked by stresses. VWM patients and VWM mouse models show ISR deregulation in brain, correlating with chronic disease development. ISR inhibition ameliorates the chronic disease in VWM mice. The subacute deteriorations have not been modeled yet. We hypothesized that ISR activation could worsen disease progression in mice and model the episodic neurological deterioration.Method: We chose to activate the ISR by subjecting wild-type (wt) and VWM mice to an isocaloric low protein diet. This model would allow us to investigate the contribution of ISR activation in subacute decline in VWM.Results: We found that the low protein diet did not significantly affect amino acid levels nor ISR levels in wt and VWM mouse brain. Our study serendipitously led to the discovery of increased levels of glycine, asparagine and Fgf21 mRNA in VWM mouse brain irrespective of the dietary protein content. Strikingly, the ISR was not activated by the low protein diet in the liver of VWM in contrast to wt mice, due to a modest ISR deregulation in this organ.Discussion: A model for subacute neurological deterioration in VWM was not established. Possibly, ISR deregulation in VWM results in reduced ISR responsiveness.

RGS2 promotes the translation of stress-associated proteins ATF4 and CHOP via its eIF2B-inhibitory domain

Regulator of G protein signaling 2 (RGS2) is upregulated by multiple forms of stress and can augment translational attenuation associated with the phosphorylation of the initiation factor eIF2, a hallmark of several stress-induced coping mechanisms. Under stress-induced translational inhibition, key factors, such as ATF4, are selectively expressed via alternative translation mechanisms. These factors are known to regulate molecular switches that control cell fate by regulating pro-survival and pro-apoptotic signals. The molecular mechanisms that balance these opposing responses to stresses are unclear. The present results suggest that RGS2 may be an important regulatory component in the cellular stress response through its translational control abilities. Previously, we have shown that RGS2 can interact with the translation initiation factor, eIF2B, and inhibit de novo protein synthesis. Here, we demonstrate that the expression of either full length RGS2 or its eIF2B-interacting domain (RGS2^eb) significantly increases levels of ATF4 and CHOP, both of which are linked to stress-induced apoptosis. Furthermore, we show that these effects are translationally regulated and independent of eIF2 phosphorylation. The present results thus point to a novel function of RGS2 in the stress response directly related to its ability to reduce global protein synthesis.

PERK signalling pathway mediates single prolonged stress-induced dysfunction of medial prefrontal cortex neurons

Post-traumatic stress disorder (PTSD) is characterized with abnormal learning and memory. Impairments in learning and memory are closely associated with apoptosis in the medial prefrontal cortex (mPFC). We previously examined the endoplasmic reticulum (ER) stress was involved in the apoptosis in the mPFC of PTSD. The PERK pathway plays the important role in the ER stress-induced apoptosis. The aim of the present study was to explore the role of PERK pathway in neuronal apoptosis in the mPFC of rat models of PTSD. We used the single prolonged stress (SPS) to mimic PTSD in rats and studied the effects of the PERK pathway in mPFC. Learning and memory behavior were examined by Morris water maze and novel object recognition tests. Apoptosis in mPFC was detected by TUNEL staining. Our results showed decreased learning memory and increased apoptosis of mPFC neurons in rats exposed to SPS. SPS exposure upregulate mRNA expressions of PERK, p-PERK, eIF2α, p-eIF2α, nuclear ATF4 and C/EBP-homologous protein (CHOP) in mPFC neurons, but the protein levels of these molecules showed difference in magnitude and time course. GSK2606414 (an antagonist of PERK) treatment successfully reversed the above changes. These results suggested that the PERK pathway mediated SPS-induced neural apoptosis in the mPFC. These findings will be helpful in understanding mPFC-related pathogenesis of PTSD.

Liver function and dysfunction - a unique window into the physiological reach of ER stress and the unfolded protein response

The unfolded protein response (UPR) improves endoplasmic reticulum (ER) protein folding in order to alleviate stress. Yet it is becoming increasingly clear that the UPR regulates processes well beyond those directly involved in protein folding, in some cases by mechanisms that fall outside the realm of canonical UPR signaling. These pathways are highly specific from one cell type to another, implying that ER stress signaling affects each tissue in a unique way. Perhaps nowhere is this more evident than in the liver, which-beyond being a highly secretory tissue-is a key regulator of peripheral metabolism and a uniquely proliferative organ upon damage. The liver provides a powerful model system for exploring how and why the UPR extends its reach into physiological processes that occur outside the ER, and how ER stress contributes to the many systemic diseases that involve liver dysfunction. This review will highlight the ways in which the study of ER stress in the liver has expanded the view of the UPR to a response that is a key guardian of cellular homeostasis outside of just the narrow realm of ER protein folding.

Resistant starch prevents tumorigenesis of dimethylhydrazine-induced colon tumors via regulation of an ER stress-mediated mitochondrial apoptosis pathway

Resistant starch is as common soluble fiber that escapes digestion in the small intestine and can regulate intestinal function, metabolism of blood glucose and lipids, and may prevent tumorigenesis of gastrointestinal cancer. Epidemiology and other evidence have suggested that resistant starch may prevent colon cancer development. The aim of the current study was to explore the ameliorative effects and potential mechanisms of resistant starch in the tumorigenesis of colon tumors induced by dimethylhydrazine in C57BL/6 mice. Western blot analysis, ELISA, microscopy, immunofluorescence and immunohistochemistry were used to analyze the efficacy of resistant starch on the metabolic balance in the colon and tumorigenesis of colon tumors. The results demonstrated that a diet containing resistant starch decreased the animal body weight and reduced free ammonia, pH and short chain fatty acids in feces compared with mice that received a standard diet. Resistant starch reduced the incidence of colon tumors and suppressed the expression of carcinogenesis-associated proteins, including heat shock protein 25, protein kinase C-d and gastrointestinal glutathione peroxidase in colon epithelial cells compared with standard starch and control groups. Colon tumor cells proliferation and dedifferentiation were significantly decreased by a resistant starch diet. The results also demonstrated that resistant starch increased the apoptosis of colon tumor cells through regulation of apoptosis-associated gene expression levels in colon tumor cells. Oxidative stress and endoplasmic reticulum stress were upregulated, and elevation eukaryotic translation initiation factor 2α (eIF2α), activating transcription factor-4 and secretase-β expression levels were increased in the resistant starch diet group. Additionally, the activity of eIF2α and PERK were increased in colon tumor cells from mice that had received resistant starch. Increasing DNA damage-inducible transcript 3 protein (CHOP), binding immunoglobulin protein (BIP) and caspase-12 expression levels upregulated by resistant starch diet may contribute to the resistant starch-induced apoptosis of colon tumor cells induced by 1,2-dimethylhydrazine. In vitro assays demonstrated that knockdown of eIF2α inhibited apoptosis of colon tumor cells isolated from mice fed with resistant starch, which also downregulated CHOP, BIP and caspase-3 expression levels compared with controls. Furthermore, long-term survival of experimental mice was prolonged by the resistant starch diet compared with the standard diet group. In conclusion, the results indicate that resistant starch in the diet may prevent carcinogenesis of colon epithelial cells, mediated by enhancing apoptosis through an endoplasmic reticulum stress-mediated mitochondrial apoptosis pathway.

Activation of cell surface GRP78 decreases endoplasmic reticulum stress and neuronal death

The unfolded protein response (UPR) is an endoplasmic reticulum (ER) -related stress conserved pathway that aims to protect cells from being overwhelmed. However, when prolonged, UPR activation converts to a death signal, which relies on its PERK-eIF2α branch. Overactivation of the UPR has been implicated in many neurological diseases, including cerebral ischaemia. Here, by using an in vivo thromboembolic model of stroke on transgenic ER stress-reporter mice and neuronal in vitro models of ischaemia, we demonstrate that ischaemic stress leads to the deleterious activation of the PERK branch of the UPR. Moreover, we show that the serine protease tissue-type plasminogen activator (tPA) can bind to cell surface Grp78 (78 kD glucose-regulated protein), leading to a decrease of the PERK pathway activation, thus a decrease of the deleterious factor CHOP, and finally promotes neuroprotection. Altogether, this work highlights a new role and a therapeutic potential of the chaperone protein Grp78 as a membrane receptor of tPA capable to prevent from ER stress overactivation.

The CCAAT/enhancer-binding protein-ATF response elements-luciferase mouse model, an innovative tool to monitor the integrated stress response pathway in vivo

PURPOSE OF REVIEW

The article highlights the recent development of an ATF4 (activating transcription factor) inducible luciferase (LUC) mouse model to monitor the integrated stress response pathway (ISR) in vivo.

RECENT FINDING

The ISR pathway plays a key role in cellular adaptation to stress and is dysregulated in numerous diseases. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 α, which leads to the recruitment of the transcription factor ATF4 to specific CCAAT/enhancer-binding protein-ATF response elements (CAREs) located in the promoters of target genes. To monitor the modulation of this pathway in the whole animal and at tissue and cellular levels, we generated a CARE-driven LUC mouse model. We validated the relevance of this model to study stress-related pathologies and recently observed the correlation between the ISR pathway induction in muscle and the occurrence of stress-induced skeletal muscle atrophy.

SUMMARY

The CARE-LUC mouse model represents an innovative tool for investigating the role of the ISR pathway in physiology and disease and opens new avenues for the development of drugs that could modify this important pathway in stress-related human diseases.

Transgenic mouse model for imaging of ATF4 translational activation-related cellular stress responses in vivo

Activating transcription factor 4 (ATF4) is a translationally activated protein that plays a role in cellular adaptation to several stresses. Because these stresses are associated with various diseases, the translational control of ATF4 needs to be evaluated from the physiological and pathological points of view. We have developed a transgenic mouse model to monitor the translational activation of ATF4 in response to cellular stress. By using this mouse model, we were able to detect nutrient starvation response, antivirus response, endoplasmic reticulum (ER) stress response, and oxidative stress in vitro and ex vivo, as well as in vivo. The reporter system introduced into our mouse model was also shown to work in a stress intensity-dependent manner and a stress duration-dependent manner. The mouse model is therefore a useful tool for imaging ATF4 translational activation at various levels, from cell cultures to whole bodies, and it has a range of useful applications in investigations on the physiological and pathological roles of ATF4-related stress and in the development of clinical drugs for treating ATF4-associated diseases.

Nutrient shortage triggers the hexosamine biosynthetic pathway via the GCN2-ATF4 signalling pathway

The hexosamine biosynthetic pathway (HBP) is a nutrient-sensing metabolic pathway that produces the activated amino sugar UDP-N-acetylglucosamine, a critical substrate for protein glycosylation. Despite its biological significance, little is known about the regulation of HBP flux during nutrient limitation. Here, we report that amino acid or glucose shortage increase GFAT1 production, the first and rate-limiting enzyme of the HBP. GFAT1 is a transcriptional target of the activating transcription factor 4 (ATF4) induced by the GCN2-eIF2α signalling pathway. The increased production of GFAT1 stimulates HBP flux and results in an increase in O-linked β-N-acetylglucosamine protein modifications. Taken together, these findings demonstrate that ATF4 provides a link between nutritional stress and the HBP for the regulation of the O-GlcNAcylation-dependent cellular signalling.