Transgenic mouse and cell culture models demonstrate a lack of mechanistic connection between endoplasmic reticulum stress and tau dysfunction

Evidence against a contribution of the CCAAT-enhancer binding protein homologous protein (CHOP) in mediating neurotoxicity in rTg4510 mice

Tau accumulation and progressive loss of neurons are associated with Alzheimer’s disease (AD). Aggregation of tau has been associated with endoplasmic reticulum (ER) stress and the activation of the unfolded protein response (UPR). While ER stress and the UPR have been linked to AD, the contribution of these pathways to tau-mediated neuronal death is still unknown. We tested the hypothesis that reducing C/EBP Homologous Protein (CHOP), a UPR induced transcription factor associated with cell death, would mitigate tau-mediated neurotoxicity through the ER stress pathway. To evaluate this, 8.5-month-old male rTg4510 tau transgenic mice were injected with a CHOP-targeting or scramble shRNA AAV9 that also expressed EGFP. Following behavioral assessment, brain tissue was collected at 12 months, when ER stress and neuronal loss is ongoing. No behavioral differences in locomotion, anxiety-like behavior, or learning and memory were found in shCHOP mice. Unexpectedly, mice expressing shCHOP had higher levels of CHOP, which did not affect neuronal count, UPR effector (ATF4), or tau tangles. Overall, this suggests that CHOP is a not a main contributor to neuronal death in rTg4510 mice. Taken together with previous studies, we conclude that ER stress, including CHOP upregulation, does not worsen outcomes in the tauopathic brain.

Molecular Mechanisms of ER Stress and UPR in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease involving aggregation of misfolded proteins inside the neuron causing prolonged cellular stress. The neuropathological hallmarks of AD include the formation of senile plaques and neurofibrillary tangles in specific brain regions that lead to synaptic loss and neuronal death. The exact mechanism of neuron dysfunction in AD remains obscure. In recent years, endoplasmic reticulum (ER) dysfunction has been implicated in neuronal degeneration seen in AD. Apart from AD, many other diseases also involve misfolded proteins aggregations in the ER, a condition referred to as ER stress. The response of the cell to ER stress is to activate a group of signaling pathways called unfolded protein response (UPR) that stimulates a particular transcriptional program to restore ER function and ensure cell survival. ER stress also involves the generation of reactive oxygen species (ROS) that, together with mitochondrial ROS and decreased effectiveness of antioxidant mechanisms, producing a condition of chronic oxidative stress. The unfolded proteins may not always produce a response that leads to the restoration of cellular functions, but they may also lead to inflammation by a set of different pathways with deleterious consequences. In this review, we extensively discuss the role of ER stress and how to target it using different pharmacological approaches in AD development and onset.

The PERK-Dependent Molecular Mechanisms as a Novel Therapeutic Target for Neurodegenerative Diseases

Higher prevalence of neurodegenerative diseases is strictly connected with progressive aging of the world population. Interestingly, a broad range of age-related, neurodegenerative diseases is characterized by a common pathological mechanism-accumulation of misfolded and unfolded proteins within the cells. Under certain circumstances, such protein aggregates may evoke endoplasmic reticulum (ER) stress conditions and subsequent activation of the unfolded protein response (UPR) signaling pathways via the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dependent manner. Under mild to moderate ER stress, UPR has a pro-adaptive role. However, severe or long-termed ER stress conditions directly evoke shift of the UPR toward its pro-apoptotic branch, which is considered to be a possible cause of neurodegeneration. To this day, there is no effective cure for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), or prion disease. Currently available treatment approaches for these diseases are only symptomatic and cannot affect the disease progression. Treatment strategies, currently under detailed research, include inhibition of the PERK-dependent UPR signaling branches. The newest data have reported that the use of small-molecule inhibitors of the PERK-mediated signaling branches may contribute to the development of a novel, ground-breaking therapeutic approach for neurodegeneration. In this review, we critically describe all the aspects associated with such targeted therapy against neurodegenerative proteopathies.

Pathogenic tau does not drive activation of the unfolded protein response

The unfolded protein response (UPR) is commonly associated with a range of neurodegenerative diseases, and targeting UPR components has been suggested as a therapeutic strategy. The UPR surveys protein folding within the endoplasmic reticulum. However, many of the misfolded proteins that accumulate in neurodegeneration are localized so that they do not directly cause endoplasmic reticulum triggers that activate this pathway. Here, using a transgenic mouse model and primary cell cultures along with quantitative PCR, immunoblotting, and immunohistochemistry, we tested whether the UPR is induced in in vivo and in vitro murine models of tauopathy that are based on expression of mutant tau^P301L We found no evidence for the UPR in the rTg4510 mouse model, in which mutant tau is transgenically expressed under the control of tetracycline-controlled transactivator protein. This observation was supported by results from acute experiments in which neuronal cultures expressed mutant tau and accumulated misfolded cytoplasmic tau aggregates but exhibited no UPR activation. These results suggest that the UPR is not induced as a response to tau misfolding and aggregation despite clear evidence for progressive cellular dysfunction and degeneration. We propose that caution is needed when evaluating the implied significance of the UPR as a critical determinant across major neurodegenerative diseases.

Critical review: involvement of endoplasmic reticulum stress in the aetiology of Alzheimer's disease

The endoplasmic reticulum (ER) stress response is regarded as an important process in the aetiology of Alzheimer's disease (AD). The accumulation of pathogenic misfolded proteins and the disruption of intracellular calcium (Ca²⁺) signalling are considered to be fundamental mechanisms that underlie the induction of ER stress, leading to neuronal cell death. Indeed, a number of studies have proposed molecular mechanisms linking ER stress to AD pathogenesis based on results from in vitro systems and AD mouse models. However, stress responsivity was largely different between each mouse model, even though all of these models display AD-related pathologies. While several reports have shown elevated ER stress responses in amyloid precursor protein (APP) and presenilin 1 (PS1) double-transgenic (Tg) AD mouse models, we and other groups, in contrast, observed no such ER stress response in APP-single-Tg or App-knockin mice. Therefore, it is debatable whether the ER stress observed in APP and PS1 double-Tg mice is due to AD pathology. From these findings, the roles of ER stress in AD pathogenesis needs to be carefully addressed in future studies. In this review, we summarize research detailing the relationship between ER stress and AD, and analyse the results in detail.

Role of Endoplasmic Reticulum Stress in Learning and Memory Impairment and Alzheimer's Disease-Like Neuropathology in the PS19 and APPSwe Mouse Models of Tauopathy and Amyloidosis

Emerging evidence suggests that endoplasmic reticulum (ER) stress may be involved in the pathogenesis of Alzheimer's disease (AD). Recently, pharmacological modulation of the eukaryotic translation initiation factor-2 (eIF2α) pathway was achieved using an integrated stress response inhibitor (ISRIB). While members of this signaling cascade have been suggested as potential therapeutic targets for neurodegeneration, the biological significance of this pathway has not been comprehensively assessed in animal models of AD. The present study investigated the ER stress pathway and its long-term modulation utilizing in vitro and in vivo experimental models of tauopathy (MAPT P301S)PS19 and amyloidosis (APP^Swe). We report that thapsigargin induces activating transcription factor-4 (ATF4) in primary cortical neurons (PCNs) derived from rat and APP^Swe nontransgenic (nTg) and transgenic (Tg) mice. ISRIB mitigated the induction of ATF4 in PCNs generated from wild-type (WT) but not APP^Swe mice despite partially restoring thapsigargin-induced translational repression in nTg PCNs. In vivo, C57BL/6J and PS19 mice received prolonged, once-daily administration of ISRIB. While the compound was well tolerated by PS19 and C57BL/6J mice, APP^Swe mice treated per this schedule displayed significant mortality. Thus, the dose was reduced and administered only on behavioral test days. ISRIB did not improve learning and memory function in APP^Swe Tg mice. While ISRIB did not reduce tau-related neuropathology in PS19 Tg mice, no evidence of ER stress-related dysfunction was observed in either of these Tg models. Taken together, the significance of ER stress and the relevance of these models to the etiology of AD require further investigation.

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

ER-stress in Alzheimer's disease: turning the scale?

Pathogenic mechanisms of Alzheimer's disease (AD) are intensely investigated as it is the most common form of dementia and burdens society by its costs and social demands. While key molecules such as A-beta peptides and tau have been identified decades ago, it is still enigmatic what drives the disease in its sporadic manifestation. Synthesis of A-beta peptides as well as phosphorylation of tau proteins comprise normal cellular functions and occur in principle in the healthy as well as in dementia-affected persons. Dyshomeostasis of Amyloid Precursor Protein (APP) cleavage, energy metabolism or kinase/phosphatase activity due to stressors has been suggested as a trigger of the disease. One way for cells to escape stress based on dysfunction of ER is the unfolded protein response - the UPR. This pathway is composed out of three different routes that differ in proteins involved, targets and consequences for cell fate: activation of transmembrane ER resident kinases IRE1-alpha and PERK or monomerization of membrane-anchored activating transcription factor 6 (ATF6) induce activation of versatile transcription factors (XBP-1, eIF2-alpha/ATF4 and ATF6 P50). These bind to specific DNA sequences on target gene promoters and on one hand attenuate general ER-prone protein synthesis and on the other equip the cell with tools to de-stress. If cells fail in stress compensation, this signaling also is able to evoke apoptosis. In this review we summarized knowledge on how APP processing and phosphorylation of tau might be influenced by ER-stress signaling. In addition, we depicted the effects UPR itself seems to have on molecules closely related to AD and describe what is known about UPR in AD animal models as well as in human patients.