Evolution of GADD34 expression after focal cerebral ischaemia

Translation Regulation by eIF2α Phosphorylation and mTORC1 Signaling Pathways in Non-Communicable Diseases (NCDs)

Non-communicable diseases (NCDs) are medical conditions that, by definition, are non-infectious and non-transmissible among people. Much of current NCDs are generally due to genetic, behavioral, and metabolic risk factors that often include excessive alcohol consumption, smoking, obesity, and untreated elevated blood pressure, and share many common signal transduction pathways. Alterations in cell and physiological signaling and transcriptional control pathways have been well studied in several human NCDs, but these same pathways also regulate expression and function of the protein synthetic machinery and mRNA translation which have been less well investigated. Alterations in expression of specific translation factors, and disruption of canonical mRNA translational regulation, both contribute to the pathology of many NCDs. The two most common pathological alterations that contribute to NCDs discussed in this review will be the regulation of eukaryotic initiation factor 2 (eIF2) by the integrated stress response (ISR) and the mammalian target of rapamycin complex 1 (mTORC1) pathways. Both pathways integrally connect mRNA translation activity to external and internal physiological stimuli. Here, we review the role of ISR control of eIF2 activity and mTORC1 control of cap-mediated mRNA translation in some common NCDs, including Alzheimer’s disease, Parkinson’s disease, stroke, diabetes mellitus, liver cirrhosis, chronic obstructive pulmonary disease (COPD), and cardiac diseases. Our goal is to provide insights that further the understanding as to the important role of translational regulation in the pathogenesis of these diseases.

Proteostasis During Cerebral Ischemia

Cerebral ischemia is a complex pathology involving a cascade of cellular mechanisms, which deregulate proteostasis and lead to neuronal death. Proteostasis refers to the equilibrium between protein synthesis, folding, transport, and protein degradation. Within the brain proteostasis plays key roles in learning and memory by controlling protein synthesis and degradation. Two important pathways are implicated in the regulation of proteostasis: the unfolded protein response (UPR) and macroautophagy (called hereafter autophagy). Both are necessary for cell survival, however, their over-activation in duration or intensity can lead to cell death. Moreover, UPR and autophagy can activate and potentiate each other to worsen the issue of cerebral ischemia. A better understanding of autophagy and ER stress will allow the development of therapeutic strategies for stroke, both at the acute phase and during recovery. This review summarizes the latest therapeutic advances implicating ER stress or autophagy in cerebral ischemia. We argue that the processes governing proteostasis should be considered together in stroke, rather than focusing either on ER stress or autophagy in isolation.

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.

Increased GADD34 in oligodendrocytes in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β (Aβ) and abnormally phosphorylated tau which contribute to endoplasmic reticulum (ER) stress. Previous studies demonstrated that Aβ and a truncated fragment of Aβ induced death of oligodendrocytes in vitro. In addition, a triple-transgenic AD mouse model exhibits significant region-specific alterations in myelination patterns at time points preceding the appearance of Aβ accumulation. The growth arrest and DNA damage protein (GADD) 34 is up-regulated in response to ER stress and regulates subunit of protein phosphatase 1 (PP1) complex that dephosphorylates eukaryotic translation initiator factor 2α (elF2α). Thus, GADD34 is known as an ER stress regulator or ER stress marker. In a recent study, GADD34 was induced in the spinal cord glial cells of an amyotrophic lateral sclerosis (ALS) mouse model. It is interesting that reduced GADD34 delayed the onset of ALS and prolonged the survival period in the mouse model. In this study, we have demonstrated that GADD34 was increased in neurons of human AD brains. Additionally, this finding was also observed in oligodendrocytes in human AD brains. Furthermore, we showed that the expression levels of GADD34 in neurons and oligodendrocytes were significantly increased in the early stage of AD in the mouse model. As oligodendrocytes were more affected in the early stages of AD in this experimental model, ER stress of Aβ oligomers may be more related to oligodendrocytes than to neurons. These results suggest that GADD34 could be a therapeutic target for preventing ER stress in neuronal cells in AD.

Endoplasmic reticulum dysfunction in neurological disease

Endoplasmic reticulum (ER) dysfunction might have an important part to play in a range of neurological disorders, including cerebral ischaemia, sleep apnoea, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, the prion diseases, and familial encephalopathy with neuroserpin inclusion bodies. Protein misfolding in the ER initiates the well studied unfolded protein response in energy-starved neurons during stroke, which is relevant to the toxic effects of reperfusion. The toxic peptide amyloid β induces ER stress in Alzheimer's disease, which leads to activation of similar pathways, whereas the accumulation of polymeric neuroserpin in the neuronal ER triggers a poorly understood ER-overload response. In other neurological disorders, such as Parkinson's and Huntington's diseases, ER dysfunction is well recognised but the mechanisms by which it contributes to pathogenesis remain unclear. By targeting components of these signalling responses, amelioration of their toxic effects and so the treatment of a range of neurodegenerative disorders might become possible.

Protein disulfide isomerase in ALS mouse glia links protein misfolding with NADPH oxidase-catalyzed superoxide production

Protein disulfide isomerase (PDI) is an oxidoreductase assisting oxidative protein folding in the endoplasmic reticulum of all types of cells, including neurons and glia. In neurodegenerative disorders, such as amyotrophic lateral sclerosis (ALS), up-regulation of PDI is an important part of unfolded protein response (UPR) that is thought to represent an adaption reaction and thereby protect the neurons. Importantly, studies on animal models of familial ALS with mutant Cu/Zn superoxide dismutase 1 (SOD1) have shown that the mutant SOD1 in astrocytes or microglia strongly regulates the progression of the disease. Here, we found an early up-regulation of PDI in microglia of transgenic (tg) mutant SOD1 mice, indicating that in addition to neurons, UPR takes place in glial cells in ALS. The observation was supported by the finding that also the expression of a UPR marker GADD34 (growth arrest and DNA damage-inducible protein) was induced in the spinal cord glia of tg mutant SOD1 mice. Because mutant SOD1 can cause sustained activation of NADPH oxidase (NOX), we investigated the role of PDI in UPR-induced NOX activation in microglia. In BV-2 microglia, UPR resulted in NOX activation with increased production of superoxide and increased release of tumor necrosis factor-α. The phenomenon was recapitulated in primary rat microglia, murine macrophages and human monocytes. Importantly, pharmacological inhibition of PDI or its down-regulation by short interfering RNAs prevented NOX activation in microglia and subsequent production of superoxide. Thus, results strongly demonstrate that UPR, caused by protein misfolding, may lead to PDI-dependent NOX activation and contribute to neurotoxicity in neurodegenerative diseases including ALS.

The application of permanent middle cerebral artery ligation in the mouse

Focal cerebral ischemia is among the most common type of stroke seen in patients. Due to the clinical significance there has been a prolonged effort to develop suitable animal models to study the events that unfold during ischemic insult. These techniques include transient or permanent, focal or global ischemia models using many different animal models, with the most common being rodents. The permanent MCA ligation method which is also referred as pMCAo in the literature is used extensively as a focal ischemia model in rodents. This method was originally described for rats by Tamura et al. in 1981. In this protocol a craniotomy was used to access the MCA and the proximal regions were occluded by electrocoagulation. The infarcts involve mostly cortical and sometimes striatal regions depending on the location of the occlusion. This technique is now well established and used in many laboratories. Early use of this technique led to the definition and description of "infarct core" and "penumbra", and it is often used to evaluate potential neuroprotective compounds. Although the initial studies were performed in rats, permanent MCA ligation has been used successfully in mice with slight modifications. This model yields reproducible infarcts and increased post-survival rates. Approximately 80% of the ischemic strokes in humans happen in the MCA area and thus this is a very relevant model for stroke studies. Currently, there is a paucity of effective treatments available to stroke patients, and thus there is a need for good models to test potential pharmacological compounds and evaluate physiological outcomes. This method can also be used for studying intracellular hypoxia response mechanisms in vivo. Here, we present the MCA ligation surgery in a C57/BL6 mouse. We describe the pre-surgical preparation, MCA ligation surgery and 2,3,5 Triphenyltetrazolium chloride (TTC) staining for quantification of infarct volumes.

Irreversible translation arrest in the reperfused brain

Irreversible translation arrest occurs in reperfused neurons that will die by delayed neuronal death. It is now recognized that suppression of protein synthesis is a general response of eukaryotic cells to exogenous stressors. Indeed, stress-induced translation arrest can be viewed as a component of cell stress responses, and consists of initiation, maintenance, and termination phases that work in concert with stress-induced transcriptional mechanisms. Within this framework, we review translation arrest in reperfused neurons. This framework provides a basis to recognize that phosphorylation of the alpha subunit of eukaryotic initiation factor 2 is the initiator of translation arrest, and a key marker indicating activation of neuronal stress responses. However, eIF2 alpha phosphorylation is reversible. Other phases of stress-induced translation arrest appear to contribute to irreversible translation arrest specifically in ischemic vulnerable neuron populations. We detail two lines of evidence supporting this view. First, ischemia, as a stress stimulus, induces irreversible co-translational protein misfolding and aggregation after 4 to 6 h of reperfusion, trapping protein synthesis machinery into functionally inactive protein aggregates. Second, ischemia and reperfusion leads to modifications of stress granules (SGs) that sequester functionally inactive 48S preinitiation complexes to maintain translation arrest. At later reperfusion durations, these mechanisms may converge such that SGs become sequestered in protein aggregates. These mechanisms result in elimination of functionally active ribosomes and preclude recovery of protein synthesis in selectively vulnerable neurons. Thus, recognizing translation arrest as a component of endogenous cellular stress response pathways will aid in making sense of the complexities of postischemic translation arrest.

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.