Role of Endoplasmic Reticulum Stress via the PERK Signaling Pathway in Brain Injury from Status Epilepticus

NUS1 Variants Cause Lennox-Gastaut Syndrome Related to Unfolded Protein Reaction Activation

NUS1 encodes the Nogo-B receptor, a critical regulator for unfolded protein reaction (UPR) signaling. Although several loss-of-function variants of NUS1 have been identified in patients with developmental and epileptic encephalopathy (DEE), the role of the NUS1 variant in Lennox-Gastaut syndrome (LGS), a severe child-onset DEE, remains unknown. In this study, we identified two de novo variants of NUS1, a missense variant (c.868 C > T/p.R290C) and a splice site variant (c.792-2 A > G), in two unrelated LGS patients using trio-based whole-exome sequencing performed in a cohort of 165 LGS patients. Both variants were absent in the gnomAD population and showed a significantly higher observed number of variants than expected genome-wide. The R290C variant was predicted to damage NUS1 and decrease its protein stability. The c.792-2 A > G variant caused premature termination of the protein. Knockdown of NUS1 activated the UPR pathway, resulting in apoptosis of HEK293T cells. Supplementing cells with expression of wild-type NUS1, but not the mutant (R290C), rescued UPR activation and apoptosis in NUS1 knockdown cells. Compared to wild-type Drosophila, seizure-like behaviors and excitability in projection neurons were significantly increased in Tango14 (homolog of human NUS1) knockdown and Tango14R290C/+ knock-in Drosophila. Additionally, abnormal development and a small body size were observed in both mutants. Activated UPR signaling was also detected in both mutants. Thus, NUS1 is a causative gene for LGS with dominant inheritance. The pathogenicity of these variants is related to the UPR signaling activation, which may be a common pathogenic mechanism of DEE.

Endoplasmic reticulum stress and its role in various neurodegenerative diseases

The Endoplasmic reticulum (ER), a critical cellular organelle, maintains cellular homeostasis by regulating calcium levels and orchestrating essential functions such as protein synthesis, folding, and lipid production. A pivotal aspect of ER function is its role in protein quality control. When misfolded proteins accumulate within the ER due to factors like protein folding chaperone dysfunction, toxicity, oxidative stress, or inflammation, it triggers the Unfolded protein response (UPR). The UPR involves the activation of chaperones like calnexin, calreticulin, glucose-regulating protein 78 (GRP78), and Glucose-regulating protein 94 (GRP94), along with oxidoreductases like protein disulphide isomerases (PDIs). Cells employ the Endoplasmic reticulum-associated degradation (ERAD) mechanism to counteract protein misfolding. ERAD disruption causes the detachment of GRP78 from transmembrane proteins, initiating a cascade involving Inositol-requiring kinase/endoribonuclease 1 (IRE1), Activating transcription factor 6 (ATF6), and Protein kinase RNA-like endoplasmic reticulum kinase (PERK) pathways. The accumulation and deposition of misfolded proteins within the cell are hallmarks of numerous neurodegenerative diseases. These aberrant proteins disrupt normal neuronal signalling and contribute to impaired cellular homeostasis, including oxidative stress and compromised protein degradation pathways. In essence, ER stress is defined as the cellular response to the accumulation of misfolded proteins in the endoplasmic reticulum, encompassing a series of signalling pathways and molecular events that aim to restore cellular homeostasis. This comprehensive review explores ER stress and its profound implications for the pathogenesis and progression of neurodegenerative diseases.

The role of molecular chaperones in the mechanisms of epileptogenesis

Epilepsy is a group of neurological diseases which requires significant economic costs for the treatment and care of patients. The central point of epileptogenesis stems from the failure of synaptic signal transmission mechanisms, leading to excessive synchronous excitation of neurons and characteristic epileptic electroencephalogram activity, in typical cases being manifested as seizures and loss of consciousness. The causes of epilepsy are extremely diverse, which is one of the reasons for the complexity of selecting a treatment regimen for each individual case and the high frequency of pharmacoresistant cases. Therefore, the search for new drugs and methods of epilepsy treatment requires an advanced study of the molecular mechanisms of epileptogenesis. In this regard, the investigation of molecular chaperones as potential mediators of epileptogenesis seems promising because the chaperones are involved in the processing and regulation of the activity of many key proteins directly responsible for the generation of abnormal neuronal excitation in epilepsy. In this review, we try to systematize current data on the role of molecular chaperones in epileptogenesis and discuss the prospects for the use of chemical modulators of various chaperone groups' activity as promising antiepileptic drugs.

The roles of ER stress in epilepsy: Molecular mechanisms and therapeutic implications

Epilepsies are a diverse group of neurological disorders, which are characterized by spontaneous recurrent seizures. Although a wide range of pathogenic mechanisms such as alterations in ion channels, inflammation and neuronal loss have been reported to be implicated in the epileptogenesis, the underlying pathogenesis of epilepsy remains unclear currently. Endoplasmic reticulum (ER) stress is regarded as a condition that unfolded or misfolded proteins accumulate in the ER lumen. Excessive or prolonged ER stress causes the activation of the unfolded protein response (UPR) to buffer ER stress and restore ER homeostasis. Increasing evidence has indicated dysregulated ER stress during epileptogenesis, which may participate in various pathological processes associated with epilepsy. In this present review, we summarized recent advances in the involvement of ER stress in the pathogenesis of epilepsy. Additionally, the antiepileptic and neuroprotective effects of interventions targeting ER stress were also discussed.

Alzheimer-like amyloid and tau alterations associated with cognitive deficit in temporal lobe epilepsy

Temporal lobe epilepsy represents a major cause of drug-resistant epilepsy. Cognitive impairment is a frequent comorbidity, but the mechanisms are not fully elucidated. We hypothesized that the cognitive impairment in drug-resistant temporal lobe epilepsy could be due to perturbations of amyloid and tau signalling pathways related to activation of stress kinases, similar to those observed in Alzheimer's disease. We examined these pathways, as well as amyloid-β and tau pathologies in the hippocampus and temporal lobe cortex of drug-resistant temporal lobe epilepsy patients who underwent temporal lobe resection (n = 19), in comparison with age- and region-matched samples from neurologically normal autopsy cases (n = 22). Post-mortem temporal cortex samples from Alzheimer's disease patients (n = 9) were used as positive controls to validate many of the neurodegeneration-related antibodies. Western blot and immunohistochemical analysis of tissue from temporal lobe epilepsy cases revealed increased phosphorylation of full-length amyloid precursor protein and its associated neurotoxic cleavage product amyloid-β*56. Pathological phosphorylation of two distinct tau species was also increased in both regions, but increases in amyloid-β1-42 peptide, the main component of amyloid plaques, were restricted to the hippocampus. Furthermore, several major stress kinases involved in the development of Alzheimer's disease pathology were significantly activated in temporal lobe epilepsy brain samples, including the c-Jun N-terminal kinase and the protein kinase R-like endoplasmic reticulum kinase. In temporal lobe epilepsy cases, hippocampal levels of phosphorylated amyloid precursor protein, its pro-amyloidogenic processing enzyme beta-site amyloid precursor protein cleaving enzyme 1, and both total and hyperphosphorylated tau expression, correlated with impaired preoperative executive function. Our study suggests that neurodegenerative and stress-related processes common to those observed in Alzheimer's disease may contribute to cognitive impairment in drug-resistant temporal lobe epilepsy. In particular, we identified several stress pathways that may represent potential novel therapeutic targets.

Profiling the Expression of Endoplasmic Reticulum Stress Associated Heat Shock Proteins in Animal Epilepsy Models

Unfolded protein response is a signaling cascade triggered by misfolded proteins in the endoplasmic reticulum. Heat shock protein H4 (HSPH4) and A5 (HSPA5) are two chaperoning proteins present within the organelle, which target misfolded peptides during prolonged stress conditions. Epileptogenic insults and epileptic seizures are a notable source of stress on cells. To investigate whether they influence expression of these chaperones, we performed immunohistochemical stainings in brains from rats that experienced a status epilepticus (SE) as a trigger of epileptogenesis and from canine epilepsy patients. Quantification of HSPA5 and HSPH4 revealed alterations in hippocampus and parahippocampal cortex. In rats, SE induced up-regulation of HSPA5 in the piriform cortex and down-regulation of HSPA5 and HSPH4 in the hippocampus. Regionally restricted increases in expression of the two proteins has been observed in the chronic phase with spontaneous recurrent seizures. Confocal microscopy revealed a predominant expression of both proteins in neurons, no expression in microglia and circumscribed expression in astroglia. In canine patients, only up-regulation of HSPH4 expression was observed in Cornu Ammonis 1 region in animals diagnosed with structural epilepsy. This characterization of HSPA5 and HSPH4 expression provided extensive information regarding spatial and temporal alterations of the two proteins during SE-induced epileptogenesis and following epilepsy manifestations. Up-regulation of both proteins implies stress exerted on ER during these disease phases. Taken together suggest a differential impact of epileptogenesis on HSPA5 and HSPH4 expression and indicate them as a possible target for pharmacological modulation of unfolded protein response.

The effect of metformin treatment on endoplasmic reticulum (ER) stress induced by status epilepticus (SE) via the PERK-eIF2α-CHOP pathway

Status epilepticus (SE) is defined as continuous seizure activity lasting more than 5 minutes. It results in neuronal cell death, mediated by endoplasmic reticulum (ER) stress response. Previously, metformin demonstrated neuroprotective effects in primary cortical neurons. In this study, we analyzed the effect of metformin on ER stress via the pro-apoptotic protein kinase RNA-like endoplasmic reticulum kinase (PERK)-eukaryotic initiation factor 2α (eIF2α)-C/EBP homologous protein (CHOP) pathway. SE was induced in rats by pentylenetetrazole. Following SE, the rats were treated with salubrinal, GSK2656157, or metformin. In a control group (normal saline) SE was not induced. CHOP, eIF2α, and PERK expression was determined by Western blot; apoptosis was analyzed by TUNEL assay. CHOP expression was significantly increased at 6 and 24 hours following SE. At both time points, eIF2α and PERK levels were also increased. At 6 hours, CHOP expression was significantly reduced in salubrinal, GSK2656157 and metformin groups versus SE group. eIF2α and PERK levels were decreased in metformin compared to SE group. eIF2α expression was markedly decreased in salubrinal versus SE group, while PERK expression was markedly reduced in GSK2656157 versus SE group. At 6 and 24 hours, the apoptosis rate was significantly increased in SE versus control group, while it was significantly reduced in salubrinal, GSK2656157, and metformin groups compared to SE group. The apoptosis rate also decreased in salubrinal group at 24 hours, although not to the extent observed in metformin group. Overall, CHOP expression and apoptosis induced by SE in rats were reduced with metformin. Further studies are required to evaluate the clinical relevance of metformin for patients with SE.

Endoplasmic reticulum (ER) stress protein responses in relation to spatio-temporal dynamics of astroglial responses to status epilepticus in rats

In the present study, we investigated whether endoplasmic reticulum (ER) stress is associated with neuronal- and astroglial-death in the hippocampus using LiCl-pilocarpine-induced status epilepticus (SE) rat model. Glucose-related protein (GRP) 78 and protein disulfide isomerase (PDI) expressions were transiently increased in CA1 neurons and dentate granule cells, and subsequently decreased in these cells following SE. GRP94 and calnexin (CNX) expression was gradually reduced in CA1 neurons, not in dentate granule cells. Phospho-protein kinase RNA (PKR)-like ER kinase (pPERK), phospho-eukaryotic initiation factor 2α (peIF2A) and CCAAT/enhancer-binding protein homologous protein (CHOP) immunoreactivities were observed in 17%, 12% and 7% of degenerating CA1 neurons, respectively. GRP 78 and PDI expressions were also up-regulated in reactive astrocytes within the CA1-3 regions. In the molecular layer of the dentate gyrus, PDI-positive astrocytes showed TUNEL signal, nuclear apoptosis inducing factor translocation and pPERK/peIF2A/CHOP immunoreactivities. Four weeks after SE, clasmatodendritic astrocytes showed pPERK peIF2A and CNX immunoreactivities without CHOP expression. These findings indicate that SE-induced ER stress may be associated with astroglial apoptosis and autophagic astroglial death in the regional-specific pattern.

Neuronal stress signaling and eIF2α phosphorylation as molecular links between Alzheimer's disease and diabetes

Mounting evidence from clinical, epidemiological, neuropathology and preclinical studies indicates that mechanisms similar to those leading to peripheral metabolic deregulation in metabolic disorders, such as diabetes and obesity, take place in the brains of Alzheimer's disease (AD) patients. These include pro-inflammatory mechanisms, brain metabolic stress and neuronal insulin resistance. From a molecular and cellular perspective, recent progress has been made in unveiling novel pathways that act in an orchestrated way to cause neuronal damage and cognitive decline in AD. These pathways converge to the activation of neuronal stress-related protein kinases and excessive phosphorylation of eukaryotic translation initiation factor 2α (eIF2α-P), which plays a key role in control of protein translation, culminating in synapse dysfunction and memory loss. eIF2α-P signaling thus links multiple neuronal stress pathways to impaired neuronal function and neurodegeneration. Here, we present a critical analysis of recently discovered molecular mechanisms underlying impaired brain insulin signaling and metabolic stress, with emphasis on the role of stress kinase/eIF2α-P signaling as a hub that promotes brain and behavioral impairments in AD. Because very similar mechanisms appear to operate in peripheral metabolic deregulation in T2D and in brain defects in AD, we discuss the concept that targeting defective brain insulin signaling and neuronal stress mechanisms with anti-diabetes agents may be an attractive approach to fight memory decline in AD. We conclude by raising core questions that remain to be addressed toward the development of much needed therapeutic approaches for AD.