Inhibition of translational initiation by activators of the glucose-regulated stress protein and heat shock protein stress response systems. Role of the interferon-inducible double-stranded RNA-activated eukaryotic initiation factor 2alpha kinase

Activation of the mitochondrial unfolded protein response regulates the dynamic formation of stress granules

To rapidly adapt to harmful changes to their environment, cells activate the integrated stress response (ISR). This results in an adaptive transcriptional and translational rewiring, and the formation of biomolecular condensates named stress granules (SGs), to resolve stress. In addition to this first line of defence, the mitochondrial unfolded protein response (UPRmt) activates a specific transcriptional programme to maintain mitochondrial homeostasis. We present evidence that the SG formation and UPRmt pathways are intertwined and communicate. UPRmt induction results in eIF2α phosphorylation and the initial and transient formation of SGs, which subsequently disassemble. The induction of GADD34 (also known as PPP1R15A) during late UPRmt protects cells from prolonged stress by impairing further assembly of SGs. Furthermore, mitochondrial functions and cellular survival are enhanced during UPRmt activation when SGs are absent, suggesting that UPRmt-induced SGs have an adverse effect on mitochondrial homeostasis. These findings point to a novel crosstalk between SGs and the UPRmt that might contribute to restoring mitochondrial functions under stressful conditions.

Alzheimer's Is a Multiform Disease of Sustained Neuronal Integrated Stress Response Driven by the C99 Fragment Generated Independently of AβPP; Proteolytic Production of Aβ Is Suppressed in AD-Affected Neurons: Evolution of a Theory

The present Perspective analyzes the remarkable evolution of the Amyloid Cascade Hypothesis 2.0 (ACH2.0) theory of Alzheimer's disease (AD) since its inception a few years ago, as reflected in the diminishing role of amyloid-beta (Aβ) in the disease. In the initial iteration of the ACH2.0, Aβ-protein-precursor (AβPP)-derived intraneuronal Aβ (iAβ), accumulated to neuronal integrated stress response (ISR)-eliciting levels, triggers AD. The neuronal ISR, in turn, activates the AβPP-independent production of its C99 fragment that is processed into iAβ, which drives the disease. The second iteration of the ACH2.0 stemmed from the realization that AD is, in fact, a disease of the sustained neuronal ISR. It introduced two categories of AD-conventional and unconventional-differing mainly in the manner of their causation. The former is caused by the neuronal ISR triggered by AβPP-derived iAβ, whereas in the latter, the neuronal ISR is elicited by stressors distinct from AβPP-derived iAβ and arising from brain trauma, viral and bacterial infections, and various types of inflammation. Moreover, conventional AD always contains an unconventional component, and in both forms, the disease is driven by iAβ generated independently of AβPP. In its third, the current, iteration, the ACH2.0 posits that proteolytic production of Aβ is suppressed in AD-affected neurons and that the disease is driven by C99 generated independently of AβPP. Suppression of Aβ production in AD seems an oxymoron: Aβ is equated with AD, and the later is inconceivable without the former in an ingrained Amyloid Cascade Hypothesis (ACH)-based notion. But suppression of Aβ production in AD-affected neurons is where the logic leads, and to follow it we only need to overcome the inertia of the preexisting assumptions. Moreover, not only is the generation of Aβ suppressed, so is the production of all components of the AβPP proteolytic pathway. This assertion is not a quantum leap (unless overcoming the inertia counts as such): the global cellular protein synthesis is severely suppressed under the neuronal ISR conditions, and there is no reason for constituents of the AβPP proteolytic pathway to be exempted, and they, apparently, are not, as indicated by the empirical data. In contrast, tau protein translation persists in AD-affected neurons under ISR conditions because the human tau mRNA contains an internal ribosomal entry site in its 5'UTR. In current mouse models, iAβ derived from AβPP expressed exogenously from human transgenes elicits the neuronal ISR and thus suppresses its own production. Its levels cannot principally reach AD pathology-causing levels regardless of the number of transgenes or the types of FAD mutations that they (or additional transgenes) carry. Since the AβPP-independent C99 production pathway is inoperative in mice, the current transgenic models have no potential for developing the full spectrum of AD pathology. What they display are only effects of the AβPP-derived iAβ-elicited neuronal ISR. The paper describes strategies to construct adequate transgenic AD models. It also details the utilization of human neuronal cells as the only adequate model system currently available for conventional and unconventional AD. The final alteration of the ACH2.0, introduced in the present Perspective, is that AβPP, which supports neuronal functionality and viability, is, after all, potentially produced in AD-affected neurons, albeit not conventionally but in an ISR-driven and -compatible process. Thus, the present narrative begins with the "omnipotent" Aβ capable of both triggering and driving the disease and ends up with this peptide largely dislodged from its pedestal and retaining its central role in triggering the disease in only one, although prevalent (conventional), category of AD (and driving it in none). Among interesting inferences of the present Perspective is the determination that "sporadic AD" is not sporadic at all ("non-familial" would be a much better designation). The term has fatalistic connotations, implying that the disease can strike at random. This is patently not the case: The conventional disease affects a distinct subpopulation, and the basis for unconventional AD is well understood. Another conclusion is that, unless prevented, the occurrence of conventional AD is inevitable given a sufficiently long lifespan. This Perspective also defines therapeutic directions not to be taken as well as auspicious ways forward. The former category includes ACH-based drugs (those interfering with the proteolytic production of Aβ and/or depleting extracellular Aβ). They are legitimate (albeit inefficient) preventive agents for conventional AD. There is, however, a proverbial snowball's chance in hell of them being effective in symptomatic AD, lecanemab, donanemab, and any other "…mab" or "…stat" notwithstanding. They comprise Aβ-specific antibodies, inhibitors of beta- and gamma-secretase, and modulators of the latter. In the latter category, among ways to go are the following: (1) Depletion of iAβ, which, if sufficiently "deep", opens up a tantalizing possibility of once-in-a-lifetime preventive transient treatment for conventional AD and aging-associated cognitive decline, AACD. (2) Composite therapy comprising the degradation of C99/iAβ and concurrent inhibition of the neuronal ISR. A single transient treatment could be sufficient to arrest the progression of conventional AD and prevent its recurrence for life. Multiple recurrent treatments would achieve the same outcome in unconventional AD. Alternatively, the sustained reduction/removal of unconventional neuronal ISR-eliciting stressors through the elimination of their source would convert unconventional AD into conventional one, preventable/treatable by a single transient administration of the composite C99/iAβ depletion/ISR suppression therapy. Efficient and suitable ISR inhibitors are available, and it is explicitly clear where to look for C99/iAβ-specific targeted degradation agents-activators of BACE1 and, especially, BACE2. Directly acting C99/iAβ-specific degradation agents such as proteolysis-targeting chimeras (PROTACs) and molecular-glue degraders (MGDs) are also viable options. (3) A circumscribed shift (either upstream or downstream) of the position of transcription start site (TSS) of the human AβPP gene, or, alternatively, a gene editing-mediated excision or replacement of a small, defined segment of its portion encoding 5'-untranslated region of AβPP mRNA; targeting AβPP RNA with anti-antisense oligonucleotides is another possibility. If properly executed, these RNA-based strategies would not interfere with the protein-coding potential of AβPP mRNA, and each would be capable of both preventing and stopping the AβPP-independent generation of C99 and thus of either preventing AD or arresting the progression of the disease in its conventional and unconventional forms. The paper is interspersed with "validation" sections: every conceptually significant notion is either validated by the existing data or an experimental procedure validating it is proposed.

Production of Amyloid-β in the Aβ-Protein-Precursor Proteolytic Pathway Is Discontinued or Severely Suppressed in Alzheimer's Disease-Affected Neurons: Contesting the 'Obvious'

A notion of the continuous production of amyloid-β (Aβ) via the proteolysis of Aβ-protein-precursor (AβPP) in Alzheimer's disease (AD)-affected neurons constitutes both a cornerstone and an article of faith in the Alzheimer's research field. The present Perspective challenges this assumption. It analyses the relevant empirical data and reaches an unexpected conclusion, namely that in AD-afflicted neurons, the production of AβPP-derived Aβ is either discontinued or severely suppressed, a concept that, if proven, would fundamentally change our understanding of the disease. This suppression, effectively self-suppression, occurs in the context of the global inhibition of the cellular cap-dependent protein synthesis as a consequence of the neuronal integrated stress response (ISR) elicited by AβPP-derived intraneuronal Aβ (iAβ; hence self-suppression) upon reaching certain levels. Concurrently with the suppression of the AβPP proteolytic pathway, the neuronal ISR activates in human neurons, but not in mouse neurons, the powerful AD-driving pathway generating the C99 fragment of AβPP independently of AβPP. The present study describes molecular mechanisms potentially involved in these phenomena, propounds novel approaches to generate transgenic animal models of AD, advocates for the utilization of human neuronal cells-based models of the disease, makes verifiable predictions, suggests experiments designed to validate the proposed concept, and considers its potential research and therapeutic implications. Remarkably, it opens up the possibility that the conventional production of AβPP, BACE enzymes, and γ-secretase components is also suppressed under the neuronal ISR conditions in AD-affected neurons, resulting in the dyshomeostasis of AβPP. It follows that whereas conventional AD is triggered by AβPP-derived iAβ accumulated to the ISR-eliciting levels, the disease, in its both conventional and unconventional (triggered by the neuronal ISR-eliciting stressors distinct from iAβ) forms, is driven not (or not only) by iAβ produced in the AβPP-independent pathway, as we proposed previously, but mainly, possibly exclusively, by the C99 fragment generated independently of AβPP and not cleaved at the γ-site due to the neuronal ISR-caused deficiency of γ-secretase (apparently, the AD-driving "substance X" predicted in our previous study), a paradigm consistent with a dictum by George Perry that Aβ is "central but not causative" in AD. The proposed therapeutic strategies would not only deplete the driver of the disease and abrogate the AβPP-independent production of C99 but also reverse the neuronal ISR and ameliorate the AβPP dyshomeostasis, a potentially significant contributor to AD pathology.

Quintessential Synergy: Concurrent Transient Administration of Integrated Stress Response Inhibitors and BACE1 and/or BACE2 Activators as the Optimal Therapeutic Strategy for Alzheimer's Disease

The present study analyzes two potential therapeutic approaches for Alzheimer's disease (AD). One is the suppression of the neuronal integrated stress response (ISR). Another is the targeted degradation of intraneuronal amyloid-beta (iAβ) via the activation of BACE1 (Beta-site Aβ-protein-precursor Cleaving Enzyme) and/or BACE2. Both approaches are rational. Both are promising. Both have substantial intrinsic limitations. However, when combined in a carefully orchestrated manner into a composite therapy they display a prototypical synergy and constitute the apparently optimal, potentially most effective therapeutic strategy for AD.

Deubiquitylase OTUD3 regulates integrated stress response to suppress progression and sorafenib resistance of liver cancer

The integrated stress response (ISR) is activated in response to intrinsic and extrinsic stimuli, playing a role in tumor progression and drug resistance. The regulatory role and mechanism of ISR in liver cancer, however, remain largely unexplored. Here, we demonstrate that OTU domain-containing protein 3 (OTUD3) is a deubiquitylase of eukaryotic initiation factor 2α (eIF2α), antagonizing ISR and suppressing liver cancer. OTUD3 decreases interactions between eIF2α and the kinase EIF2ΑK3 by removing K27-linked polyubiquitylation on eIF2α. OTUD3 deficiency in mice leads to enhanced ISR and accelerated progression of N-nitrosodiethylamine-induced hepatocellular carcinoma. Additionally, decreased OTUD3 expression associated with elevated eIF2α phosphorylation correlates with the progression of human liver cancer. Moreover, ISR activation due to decreased OTUD3 expression renders liver cancer cells resistant to sorafenib, while the combined use of the ISR inhibitor ISRIB significantly improves their sensitivity to sorafenib. Collectively, these findings illuminate the regulatory mechanism of ISR in liver cancer and provide a potential strategy to counteract sorafenib resistance.

ACH2.0/E, the Consolidated Theory of Conventional and Unconventional Alzheimer's Disease: Origins, Progression, and Therapeutic Strategies

The centrality of amyloid-beta (Aβ) is an indisputable tenet of Alzheimer's disease (AD). It was initially indicated by the detection (1991) of a mutation within Aβ protein precursor (AβPP) segregating with the disease, which served as a basis for the long-standing Amyloid Cascade Hypothesis (ACH) theory of AD. In the intervening three decades, this notion was affirmed and substantiated by the discovery of numerous AD-causing and AD-protective mutations with all, without an exception, affecting the structure, production, and intraneuronal degradation of Aβ. The ACH postulated that the disease is caused and driven by extracellular Aβ. When it became clear that this is not the case, and the ACH was largely discredited, a new theory of AD, dubbed ACH2.0 to re-emphasize the centrality of Aβ, was formulated. In the ACH2.0, AD is caused by physiologically accumulated intraneuronal Aβ (iAβ) derived from AβPP. Upon reaching the critical threshold, it triggers activation of the autonomous AβPP-independent iAβ generation pathway; its output is retained intraneuronally and drives the AD pathology. The bridge between iAβ derived from AβPP and that generated independently of AβPP is the neuronal integrated stress response (ISR) elicited by the former. The ISR severely suppresses cellular protein synthesis; concurrently, it activates the production of a small subset of proteins, which apparently includes components necessary for operation of the AβPP-independent iAβ generation pathway that are absent under regular circumstances. The above sequence of events defines "conventional" AD, which is both caused and driven by differentially derived iAβ. Since the ISR can be elicited by a multitude of stressors, the logic of the ACH2.0 mandates that another class of AD, referred to as "unconventional", has to occur. Unconventional AD is defined as a disease where a stressor distinct from AβPP-derived iAβ elicits the neuronal ISR. Thus, the essence of both, conventional and unconventional, forms of AD is one and the same, namely autonomous, self-sustainable, AβPP-independent production of iAβ. What distinguishes them is the manner of activation of this pathway, i.e., the mode of causation of the disease. In unconventional AD, processes occurring at locations as distant from and seemingly as unrelated to the brain as, say, the knee can potentially trigger the disease. The present study asserts that these processes include traumatic brain injury (TBI), chronic traumatic encephalopathy, viral and bacterial infections, and a wide array of inflammatory conditions. It considers the pathways which are common to all these occurrences and culminate in the elicitation of the neuronal ISR, analyzes the dynamics of conventional versus unconventional AD, shows how the former can morph into the latter, explains how a single TBI can hasten the occurrence of AD and why it takes multiple TBIs to trigger the disease, and proposes the appropriate therapeutic strategies. It posits that yet another class of unconventional AD may occur where the autonomous AβPP-independent iAβ production pathway is initiated by an ISR-unrelated activator, and consolidates the above notions in a theory of AD, designated ACH2.0/E (for expanded ACH2.0), which incorporates the ACH2.0 as its special case and retains the centrality of iAβ produced independently of AβPP as the driving agent of the disease.

On the Inadequacy of the Current Transgenic Animal Models of Alzheimer's Disease: The Path Forward

For at least two reasons, the current transgenic animal models of Alzheimer's disease (AD) appear to be patently inadequate. They may be useful in many respects, the AD models; however, they are not. First, they are incapable of developing the full spectrum of the AD pathology. Second, they respond spectacularly well to drugs that are completely ineffective in the treatment of symptomatic AD. These observations indicate that both the transgenic animal models and the drugs faithfully reflect the theory that guided the design and development of both, the amyloid cascade hypothesis (ACH), and that both are inadequate because their underlying theory is. This conclusion necessitated the formulation of a new, all-encompassing theory of conventional AD-the ACH2.0. The two principal attributes of the ACH2.0 are the following. One, in conventional AD, the agent that causes the disease and drives its pathology is the intraneuronal amyloid-β (iAβ) produced in two distinctly different pathways. Two, following the commencement of AD, the bulk of Aβ is generated independently of Aβ protein precursor (AβPP) and is retained inside the neuron as iAβ. Within the framework of the ACH2.0, AβPP-derived iAβ accumulates physiologically in a lifelong process. It cannot reach levels required to support the progression of AD; it does, however, cause the disease. Indeed, conventional AD occurs if and when the levels of AβPP-derived iAβ cross the critical threshold, elicit the neuronal integrated stress response (ISR), and trigger the activation of the AβPP-independent iAβ generation pathway; the disease commences only when this pathway is operational. The iAβ produced in this pathway reaches levels sufficient to drive the AD pathology; it also propagates its own production and thus sustains the activity of the pathway and perpetuates its operation. The present study analyzes the reason underlying the evident inadequacy of the current transgenic animal models of AD. It concludes that they model, in fact, not Alzheimer's disease but rather the effects of the neuronal ISR sustained by AβPP-derived iAβ, that this is due to the lack of the operational AβPP-independent iAβ production pathway, and that this mechanism must be incorporated into any successful AD model faithfully emulating the disease. The study dissects the plausible molecular mechanisms of the AβPP-independent iAβ production and the pathways leading to their activation, and introduces the concept of conventional versus unconventional Alzheimer's disease. It also proposes the path forward, posits the principles of design of productive transgenic animal models of the disease, and describes the molecular details of their construction.

Next Generation Therapeutic Strategy for Treatment and Prevention of Alzheimer's Disease and Aging-Associated Cognitive Decline: Transient, Once-in-a-Lifetime-Only Depletion of Intraneuronal Aβ (iAβ) by Its Targeted Degradation via Augmentation of Intra-iAβ-Cleaving Activities of BACE1 and/or BACE2

Although the long-standing Amyloid Cascade Hypothesis (ACH) has been largely discredited, its main attribute, the centrality of amyloid-beta (Aβ) in Alzheimer's disease (AD), remains the cornerstone of any potential interpretation of the disease: All known AD-causing mutations, without a single exception, affect, in one way or another, Aβ. The ACH2.0, a recently introduced theory of AD, preserves this attribute but otherwise differs fundamentally from the ACH. It posits that AD is a two-stage disorder where both stages are driven by intraneuronal (rather than extracellular) Aβ (iAβ) albeit of two distinctly different origins. The first asymptomatic stage is the decades-long accumulation of Aβ protein precursor (AβPP)-derived iAβ to the critical threshold. This triggers the activation of the self-sustaining AβPP-independent iAβ production pathway and the commencement of the second, symptomatic AD stage. Importantly, Aβ produced independently of AβPP is retained intraneuronally. It drives the AD pathology and perpetuates the operation of the pathway; continuous cycles of the iAβ-stimulated propagation of its own AβPP-independent production constitute an engine that drives AD, the AD Engine. It appears that the dynamics of AβPP-derived iAβ accumulation is the determining factor that either drives Aging-Associated Cognitive Decline (AACD) and triggers AD or confers the resistance to both. Within the ACH2.0 framework, the ACH-based drugs, designed to lower levels of extracellular Aβ, could be applicable in the prevention of AD and treatment of AACD because they reduce the rate of accumulation of AβPP-derived iAβ. The present study analyzes their utility and concludes that it is severely limited. Indeed, their short-term employment is ineffective, their long-term engagement is highly problematic, their implementation at the symptomatic stages of AD is futile, and their evaluation in conventional clinical trials for the prevention of AD is impractical at best, impossible at worst, and misleading in between. In contrast, the ACH2.0-guided Next Generation Therapeutic Strategy for the treatment and prevention of both AD and AACD, namely the depletion of iAβ via its transient, short-duration, targeted degradation by the novel ACH2.0-based drugs, has none of the shortcomings of the ACH-based drugs. It is potentially highly effective, easily evaluable in clinical trials, and opens up the possibility of once-in-a-lifetime-only therapeutic intervention for prevention and treatment of both conditions. It also identifies two plausible ACH2.0-based drugs: activators of physiologically occurring intra-iAβ-cleaving capabilities of BACE1 and/or BACE2.

The Upstream 1350~1250 Nucleotide Sequences of the Human ENDOU-1 Gene Contain Critical Cis-Elements Responsible for Upregulating Its Transcription during ER Stress

ENDOU-1 encodes an endoribonuclease that overcomes the inhibitory upstream open reading frame (uORF)-trap at 5'-untranslated region (UTR) of the CHOP transcript, allowing the downstream coding sequence of CHOP be translated during endoplasmic reticulum (ER) stress. However, transcriptional control of ENDOU-1 remains enigmatic. To address this, we cloned an upstream 2.1 kb (-2055~+77 bp) of human ENDOU-1 (pE2.1p) fused with reporter luciferase (luc) cDNA. The promoter strength driven by pE2.1p was significantly upregulated in both pE2.1p-transfected cells and pE2.1p-injected zebrafish embryos treated with stress inducers. Comparing the luc activities driven by pE2.1p and -1125~+77 (pE1.2p) segments, we revealed that cis-elements located at the -2055~-1125 segment might play a critical role in ENDOU-1 upregulation during ER stress. Since bioinformatics analysis predicted many cis-elements clustered at the -1850~-1250, we further deconstructed this segment to generate pE2.1p-based derivatives lacking -1850~-1750, -1749~-1650, -1649~-1486, -1485~-1350 or -1350~-1250 segments. Quantification of promoter activities driven by these five internal deletion plasmids suggested a repressor binding element within the -1649~-1486 and an activator binding element within the -1350~-1250. Since luc activities driven by the -1649~-1486 were not significantly different between normal and stress conditions, we herein propose that the stress-inducible activator bound at the -1350~-1250 segment makes a major contribution to the increased expression of human ENDOU-1 upon ER stresses.

A Solitary Stalled 80S Ribosome Prevents mRNA Recruitment to Stress Granules

In response to stress stimuli, eukaryotic cells typically suppress protein synthesis. This leads to the release of mRNAs from polysomes, their condensation with RNA-binding proteins, and the formation of non-membrane-bound cytoplasmic compartments called stress granules (SGs). SGs contain 40S but generally lack 60S ribosomal subunits. It is known that cycloheximide, emetine, and anisomycin, the ribosome inhibitors that block the progression of 80S ribosomes along mRNA and stabilize polysomes, prevent SG assembly. Conversely, puromycin, which induces premature termination, releases mRNA from polysomes and stimulates the formation of SGs. The same effect is caused by some translation initiation inhibitors, which lead to polysome disassembly and the accumulation of mRNAs in the form of stalled 48S preinitiation complexes. Based on these and other data, it is believed that the trigger for SG formation is the presence of mRNA with extended ribosome-free segments, which tend to form condensates in the cell. In this study, we evaluated the ability of various small-molecule translation inhibitors to block or stimulate the assembly of SGs under conditions of severe oxidative stress induced by sodium arsenite. Contrary to expectations, we found that ribosome-targeting elongation inhibitors of a specific type, which arrest solitary 80S ribosomes at the beginning of the mRNA coding regions but do not interfere with all subsequent ribosomes in completing translation and leaving the transcripts (such as harringtonine, lactimidomycin, or T-2 toxin), completely prevent the formation of arsenite-induced SGs. These observations suggest that the presence of even a single 80S ribosome on mRNA is sufficient to prevent its recruitment into SGs, and the presence of extended ribosome-free regions of mRNA is not sufficient for SG formation. We propose that mRNA entry into SGs may be mediated by specific contacts between RNA-binding proteins and those regions on 40S subunits that remain inaccessible when ribosomes are associated.

Mechanisms of genotoxicity and proteotoxicity induced by the metalloids arsenic and antimony

Arsenic and antimony are metalloids with profound effects on biological systems and human health. Both elements are toxic to cells and organisms, and exposure is associated with several pathological conditions including cancer and neurodegenerative disorders. At the same time, arsenic- and antimony-containing compounds are used in the treatment of multiple diseases. Although these metalloids can both cause and cure disease, their modes of molecular action are incompletely understood. The past decades have seen major advances in our understanding of arsenic and antimony toxicity, emphasizing genotoxicity and proteotoxicity as key contributors to pathogenesis. In this review, we highlight mechanisms by which arsenic and antimony cause toxicity, focusing on their genotoxic and proteotoxic effects. The mechanisms used by cells to maintain proteostasis during metalloid exposure are also described. Furthermore, we address how metalloid-induced proteotoxicity may promote neurodegenerative disease and how genotoxicity and proteotoxicity may be interrelated and together contribute to proteinopathies. A deeper understanding of cellular toxicity and response mechanisms and their links to pathogenesis may promote the development of strategies for both disease prevention and treatment.

Principles of Design of Clinical Trials for Prevention and Treatment of Alzheimer's Disease and Aging-Associated Cognitive Decline in the ACH2.0 Perspective: Potential Outcomes, Challenges, and Solutions

With the Amyloid Cascade Hypothesis (ACH) largely discredited, the ACH2.0 theory of Alzheimer's disease (AD) has been recently introduced. Within the framework of the ACH2.0, AD is triggered by amyloid-β protein precursor (AβPP)-derived intraneuronal Aβ (iAβ) and is driven by iAβ produced in the AβPP-independent pathway and retained intraneuronally. In this paradigm, the depletion of extracellular Aβ or suppression of Aβ production by AβPP proteolysis, the two sources of AβPP-derived iAβ, would be futile in symptomatic AD, due to its reliance on iAβ generated independently of AβPP, but effective in preventing AD and treating Aging-Associated Cognitive Decline (AACD) driven, in the ACH2.0 framework, by AβPP-derived iAβ. The observed effect of lecanemab and donanemab, interpreted in the ACH2.0 perspective, supports this notion and mandates AD-preventive clinical trials. Such trials are currently in progress. They are likely, however, to fail or to yield deceptive results if conducted conventionally. The present study considers concepts of design of clinical trials of lecanemab, donanemab, or any other drug, targeting the influx of AβPP-derived iAβ, in prevention of AD and treatment of AACD. It analyzes possible outcomes and explains why selection of high-risk asymptomatic participants seems reasonable but is not. It argues that outcomes of such AD preventive trials could be grossly misleading, discusses inevitable potential problems, and proposes feasible solutions. It advocates the initial evaluation of this type of drugs in clinical trials for treatment of AACD. Whereas AD protective trials of these drugs are potentially of an impractical length, AACD clinical trials are expected to yield unequivocal results within a relatively short duration. Moreover, success of the latter, in addition to its intrinsic value, would constitute a proof of concept for the former. Furthermore, this study introduces concepts of the active versus passive iAβ depletion, contends that targeted degradation of iAβ is the best therapeutic strategy for both prevention and treatment of AD and AACD, proposes potential iAβ-degrading drugs, and describes their feasible and unambiguous evaluation in clinical trials.

The Amyloid Cascade Hypothesis 2.0 for Alzheimer's Disease and Aging-Associated Cognitive Decline: From Molecular Basis to Effective Therapy

With the long-standing amyloid cascade hypothesis (ACH) largely discredited, there is an acute need for a new all-encompassing interpretation of Alzheimer's disease (AD). Whereas such a recently proposed theory of AD is designated ACH2.0, its commonality with the ACH is limited to the recognition of the centrality of amyloid-β (Aβ) in the disease, necessitated by the observation that all AD-causing mutations affect, in one way or another, Aβ. Yet, even this narrow commonality is superficial since AD-causing Aβ of the ACH differs distinctly from that specified in the ACH2.0: Whereas in the former, the disease is caused by secreted extracellular Aβ, in the latter, it is triggered by Aβ-protein-precursor (AβPP)-derived intraneuronal Aβ (iAβ) and driven by iAβ generated independently of AβPP. The ACH2.0 envisions AD as a two-stage disorder. The first, asymptomatic stage is a decades-long accumulation of AβPP-derived iAβ, which occurs via internalization of secreted Aβ and through intracellular retention of a fraction of Aβ produced by AβPP proteolysis. When AβPP-derived iAβ reaches critical levels, it activates a self-perpetuating AβPP-independent production of iAβ that drives the second, devastating AD stage, a cascade that includes tau pathology and culminates in neuronal loss. The present study analyzes the dynamics of iAβ accumulation in health and disease and concludes that it is the prime factor driving both AD and aging-associated cognitive decline (AACD). It discusses mechanisms potentially involved in AβPP-independent generation of iAβ, provides mechanistic interpretations for all principal aspects of AD and AACD including the protective effect of the Icelandic AβPP mutation, the early onset of FAD and the sequential manifestation of AD pathology in defined regions of the affected brain, and explains why current mouse AD models are neither adequate nor suitable. It posits that while drugs affecting the accumulation of AβPP-derived iAβ can be effective only protectively for AD, the targeted degradation of iAβ is the best therapeutic strategy for both prevention and effective treatment of AD and AACD. It also proposes potential iAβ-degrading drugs.

Reversible protein assemblies in the proteostasis network in health and disease

While proteins populating their native conformations constitute the functional entities of cells, protein aggregates are traditionally associated with cellular dysfunction, stress and disease. During recent years, it has become clear that large aggregate-like protein condensates formed via liquid-liquid phase separation age into more solid aggregate-like particles that harbor misfolded proteins and are decorated by protein quality control factors. The constituent proteins of the condensates/aggregates are disentangled by protein disaggregation systems mainly based on Hsp70 and AAA ATPase Hsp100 chaperones prior to their handover to refolding and degradation systems. Here, we discuss the functional roles that condensate formation/aggregation and disaggregation play in protein quality control to maintain proteostasis and why it matters for understanding health and disease.

Dysregulation of Stress-Induced Translational Control by Porphyromonas gingivalis in Host Cells

Porphyromonas gingivalis contributes to the chronic oral disease periodontitis, triggering the activation of host inflammatory responses, inducing cellular stresses such as oxidation. During stress, host cells can activate the Integrated Stress Response (ISR), a pathway which determines cellular fate, by either downregulating protein synthesis and initiating a stress–response gene expression program, or by initiating programmed cell death. Recent studies have implicated the ISR within both host antimicrobial defenses and the pathomechanism of certain microbes. In this study, using a combination of immunofluorescence confocal microscopy and immunoblotting, the molecular mechanisms by which P. gingivalis infection alters translation attenuation during oxidative stress-induced activation of the ISR in oral epithelial cells were investigated. P. gingivalis infection alone did not result in ISR activation. In contrast, infection coupled with stress caused differential stress granule formation and composition. Infection heightened stress-induced translational repression independently of core ISR mediators. Heightened translational repression during stress was observed with both P. gingivalis–conditioned media and outer membrane vesicles, implicating a secretory factor in this exacerbated translational repression. The effects of gingipain inhibitors and gingipain-deficient P. gingivalis mutants confirmed these pathogen-specific proteases as the effector of exacerbated translational repression. Gingipains are known to degrade the mammalian target of rapamycin (mTOR) and the findings of this study implicate the gingipain-mTOR axis as the effector of host translational dysregulation during stress.

The Amyloid Cascade Hypothesis 2.0: Generalization of the Concept

Recently, we proposed the Amyloid Cascade Hypothesis 2.0 (ACH2.0), a reformulation of the ACH. In the former, in contrast to the latter, Alzheimer's disease (AD) is driven by intraneuronal amyloid-β (iAβ) and occurs in two stages. In the first, relatively benign stage, Aβ protein precursor (AβPP)-derived iAβ activates, upon reaching a critical threshold, the AβPP-independent iAβ-generating pathway, triggering a devastating second stage resulting in neuronal death. While the ACH2.0 remains aligned with the ACH premise that Aβ is toxic, the toxicity is exerted because of intra- rather than extracellular Aβ. In this framework, a once-in-a-lifetime-only iAβ depletion treatment via transient activation of BACE1 and/or BACE2 (exploiting their Aβ-cleaving activities) or by any means appears to be the best therapeutic strategy for AD. Whereas the notion of differentially derived iAβ being the principal moving force at both AD stages is both plausible and elegant, a possibility remains that the second AD stage is enabled by an AβPP-derived iAβ-activated self-sustaining mechanism producing a yet undefined deleterious "substance X" (sX) which anchors the second AD stage. The present study generalizes the ACH2.0 by incorporating this possibility and shows that, in this scenario, the iAβ depletion therapy may be ineffective at symptomatic AD stages but fully retains its preventive potential for both AD and the aging-associated cognitive decline, which is defined in the ACH2.0 framework as the extended first stage of AD.

Viral Hemorrhagic Septicemia Virus Activates Integrated Stress Response Pathway and Induces Stress Granules to Regulate Virus Replication

Virus infection activates integrated stress response (ISR) and stress granule (SG) formation and viruses counteract by interfering with SG assembly, suggesting an important role in antiviral defense. The infection of fish cells by Viral Hemorrhagic Septicemia Virus (VHSV), activates the innate immune recognition pathway and the production of type I interferon (IFN). However, the mechanisms by which VHSV interacts with ISR pathway regulating SG formation is poorly understood. Here, we demonstrate that fish cells respond to heat shock, oxidative stress and VHSV infection by forming SG that localized key SG marker, Ras GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1). We show that PKR-like endoplasmic reticulum kinase (PERK), but not (dsRNA)-dependent protein kinase (PKR), is required for VHSV-induced SG formation. Furthermore, in VHSV Ia infected cells, PERK activity is required for IFN production, antiviral signaling and viral replication. SG formation required active virus replication as individual VHSV Ia proteins or inactive virus did not induce SG. Cells lacking G3BP1 produced increased IFN, antiviral genes and viral mRNA, however viral protein synthesis and viral titers were reduced. We show a critical role of the activation of ISR pathway and SG formation highlighting a novel role of G3BP1 in regulating VHSV protein translation and replication.

Relocalization of Translation Termination and Ribosome Recycling Factors to Stress Granules Coincides with Elevated Stop-Codon Readthrough and Reinitiation Rates upon Oxidative Stress

Upon oxidative stress, mammalian cells rapidly reprogram their translation. This is accompanied by the formation of stress granules (SGs), cytoplasmic ribonucleoprotein condensates containing untranslated mRNA molecules, RNA-binding proteins, 40S ribosomal subunits, and a set of translation initiation factors. Here we show that arsenite-induced stress causes a dramatic increase in the stop-codon readthrough rate and significantly elevates translation reinitiation levels on uORF-containing and bicistronic mRNAs. We also report the recruitment of translation termination factors eRF1 and eRF3, as well as ribosome recycling and translation reinitiation factors ABCE1, eIF2D, MCT-1, and DENR to SGs upon arsenite treatment. Localization of these factors to SGs may contribute to a rapid resumption of mRNA translation after stress relief and SG disassembly. It may also suggest the presence of post-termination, recycling, or reinitiation complexes in SGs. This new layer of translational control under stress conditions, relying on the altered spatial distribution of translation factors between cellular compartments, is discussed.

The Amyloid Cascade Hypothesis 2.0: On the Possibility of Once-in-a-Lifetime-Only Treatment for Prevention of Alzheimer’s Disease and for Its Potential Cure at Symptomatic Stages

We posit that Alzheimer’s disease (AD) is driven by amyloid-β (Aβ) generated in the amyloid-β protein precursor (AβPP) independent pathway activated by AβPP-derived Aβ accumulated intraneuronally in a life-long process. This interpretation constitutes the Amyloid Cascade Hypothesis 2.0 (ACH2.0). It defines a tandem intraneuronal-Aβ (iAβ)-anchored cascade occurrence: intraneuronally-accumulated, AβPP-derived iAβ triggers relatively benign cascade that activates the AβPP-independent iAβ-generating pathway, which, in turn, initiates the second, devastating cascade that includes tau pathology and leads to neuronal loss. The entire output of the AβPP-independent iAβ-generating pathway is retained intraneuronally and perpetuates the pathway’s operation. This process constitutes a self-propagating, autonomous engine that drives AD and ultimately kills its host cells. Once activated, the AD Engine is self-reliant and independent from Aβ production in the AβPP proteolytic pathway; operation of the former renders the latter irrelevant to the progression of AD and brands its manipulation for therapeutic purposes, such as BACE (beta-site AβPP-cleaving enzyme) inhibition, as futile. In the proposed AD paradigm, the only valid direct therapeutic strategy is targeting the engine’s components, and the most effective feasible approach appears to be the activation of BACE1 and/or of its homolog BACE2, with the aim of exploiting their Aβ-cleaving activities. Such treatment would collapse the iAβ population and ‘reset’ its levels below those required for the operation of the AD Engine. Any sufficiently selective iAβ-depleting treatment would be equally effective. Remarkably, this approach opens the possibility of a short-duration, once-in-a-lifetime-only or very infrequent, preventive or curative therapy for AD; this therapy would be also effective for prevention and treatment of the ‘common’ pervasive aging-associated cognitive decline. The ACH2.0 clarifies all ACH-unresolved inconsistencies, explains the widespread ‘resilience to AD’ phenomenon, predicts occurrences of a category of AD-afflicted individuals without excessive Aβ plaque load and of a novel type of familial insusceptibility to AD; it also predicts the lifespan-dependent inevitability of AD in humans if untreated preventively. The article details strategy and methodology to generate an adequate AD model and validate the hypothesis; the proposed AD model may also serve as a research and drug development platform.

KEPI plays a negative role in the repression that accompanies translational inhibition guided by the uORF element of human CHOP transcript during stress response

Translation of the downstream coding sequence of some mRNAs may be repressed by the upstream open reading frame (uORF) at their 5'-end. The mechanism underlying this uORF-mediated translational inhibition (uORF-MTI) is not fully understood in vivo. Recently, it was found that zebrafish Endouc or its human orthologue ENDOU (Endouc/ENDOU) plays a positive role in repressing the uORF-MTI of human CHOP (uORF^chop-MTI) during stress by blocking its activity However, the repression of uORF^chop-MTI assisted by an as-yet unidentified negative effector remains to be elucidated. Compared to the upregulated CHOP transcript, we herein report that the kepi (kinase-enhanced PP1 inhibitor) transcript was downregulated in the zebrafish embryos treated with both heat shock and hypoxia. Quantitative RT-PCR also revealed that the level of kepi mRNA was noticeably decreased in both heat-shock-treated and hypoxia-exposed embryos. When kepi mRNA was microinjected into the one-celled embryos from transgenic line huORFZ, the translation of downstream GFP reporter controlled by the uORF^chop-MTI was reduced in the hypoxia-exposed embryos. In contrast, when kepi was knocked down by injection of antisense Morpholino oligonucleotide, the translation of downstream GFP reporter was induced and expressed in the brain and spinal cord of injected embryos in the absence of stress. During normal condition, overexpression of KEPI increased eIF2α phosphorylation, resulting in inducing the translation of uORF-tag mRNA, such as ATF4 and CHOP mRNAs. However, during stress condition, overexpression of KEPI decreased eIF2α phosphorylation, resulting in reducing the GFP reporter and CHOP proteins. This is the first report to demonstrate that KEPI plays a negative role in uORF^chop - mediated translation during ER stress.

Perspective: Modulating the integrated stress response to slow aging and ameliorate age-related pathology

Healthy aging requires the coordination of numerous stress signaling pathways that converge on the protein homeostasis network. The Integrated Stress Response (ISR) is activated by diverse stimuli, leading to phosphorylation of the eukaryotic translation initiation factor elF2 in its α-subunit. Under replete conditions, elF2 orchestrates 5' cap-dependent mRNA translation and is thus responsible for general protein synthesis. elF2α phosphorylation, the key event of the ISR, reduces global mRNA translation while enhancing the expression of a signature set of stress response genes. Despite the critical role of protein quality control in healthy aging and in numerous longevity pathways, the role of the ISR in longevity remains largely unexplored. ISR activity increases with age, suggesting a potential link with the aging process. Although decreased protein biosynthesis, which occurs during ISR activation, have been linked to lifespan extension, recent data show that lifespan is limited by the ISR as its inhibition extends survival in nematodes and enhances cognitive function in aged mice. Here we survey how aging affects the ISR, the role of the ISR in modulating aging, and pharmacological interventions to tune the ISR. Finally, we will explore the ISR as a plausible target for clinical interventions in aging and age-related disease.

Ubiquitination is essential for recovery of cellular activities after heat shock

Eukaryotic cells respond to stress through adaptive programs that include reversible shutdown of key cellular processes, the formation of stress granules, and a global increase in ubiquitination. The primary function of this ubiquitination is thought to be for tagging damaged or misfolded proteins for degradation. Here, working in mammalian cultured cells, we found that different stresses elicited distinct ubiquitination patterns. For heat stress, ubiquitination targeted specific proteins associated with cellular activities that are down-regulated during stress, including nucleocytoplasmic transport and translation, as well as stress granule constituents. Ubiquitination was not required for the shutdown of these processes or for stress granule formation but was essential for the resumption of cellular activities and for stress granule disassembly. Thus, stress-induced ubiquitination primes the cell for recovery after heat stress.

PDGFA-associated protein 1 protects mature B lymphocytes from stress-induced cell death and promotes antibody gene diversification

The establishment of protective humoral immunity is dependent on the ability of mature B cells to undergo antibody gene diversification while adjusting to the physiological stressors induced by activation with the antigen. Mature B cells diversify their antibody genes by class switch recombination (CSR) and somatic hypermutation (SHM), which are both dependent on efficient induction of activation-induced cytidine deaminase (AID). Here, we identified PDGFA-associated protein 1 (Pdap1) as an essential regulator of cellular homeostasis in mature B cells. Pdap1 deficiency leads to sustained expression of the integrated stress response (ISR) effector activating transcription factor 4 (Atf4) and induction of the ISR transcriptional program, increased cell death, and defective AID expression. As a consequence, loss of Pdap1 reduces germinal center B cell formation and impairs CSR and SHM. Thus, Pdap1 protects mature B cells against chronic ISR activation and ensures efficient antibody diversification by promoting their survival and optimal function.

Stress Granules in the Post-transcriptional Regulation of Immune Cells

Immune cell activation triggers transcriptional and translational programs eliciting cellular processes, such as differentiation or proliferation, essential for an efficient immune response. These dynamic processes require an intricate orchestration of regulatory mechanisms to control the precise spatiotemporal expression of proteins. Post-transcriptional regulation ensures the control of messenger RNA metabolism and appropriate translation. Among these post-transcriptional regulatory mechanisms, stress granules participate in the control of protein synthesis. Stress granules are ribonucleoprotein complexes that form upon stress, typically under control of the integrated stress response. Such structures assemble upon stimulation of immune cells where they control selective translational programs ensuring the establishment of accurate effector functions. In this review, we summarize the current knowledge about post-transcriptional regulation in immune cells and highlight the role of stress sensors and stress granules in such regulation.

Protein Stability Buffers the Cost of Translation Attenuation following eIF2α Phosphorylation

Phosphorylation of the translation initiation factor eIF2α is a rapid and vital response to many forms of stress, including protein-misfolding stress in the endoplasmic reticulum (ER stress). It is believed to cause a general reduction in protein synthesis while enabling translation of few transcripts. Such a reduction of protein synthesis comes with the threat of depleting essential proteins, a risk thought to be mitigated by its transient nature. Here, we find that translation attenuation is not uniform, with cytosolic and mitochondrial ribosomal subunits being prominently downregulated. Translation attenuation of these targets persists after translation recovery. Surprisingly, this occurs without a measurable decrease in ribosomal proteins. Explaining this conundrum, translation attenuation preferentially targets long-lived proteins, a finding not only demonstrated by ribosomal proteins but also observed at a global level. This shows that protein stability buffers the cost of translational attenuation, establishing an evolutionary principle of cellular robustness.

Integrated stress response in hepatitis C promotes Nrf2-related chaperone-mediated autophagy: A novel mechanism for host-microbe survival and HCC development in liver cirrhosis

The molecular mechanism(s) how liver damage during the chronic hepatitis C virus (HCV) infection evolve into cirrhosis and hepatocellular carcinoma (HCC) is unclear. HCV infects hepatocyte, the major cell types in the liver. During infection, large amounts of viral proteins and RNA replication intermediates accumulate in the endoplasmic reticulum (ER) of the infected hepatocyte, which creates a substantial amount of stress response. Infected hepatocyte activates a different type of stress adaptive mechanisms such as unfolded protein response (UPR), antioxidant response (AR), and the integrated stress response (ISR) to promote virus-host cell survival. The hepatic stress is also amplified by another layer of innate and inflammatory response associated with cellular sensing of virus infection through the production of interferon (IFN) and inflammatory cytokines. The interplay between various types of cellular stress signal leads to different forms of cell death such as apoptosis, necrosis, and autophagy depending on the intensity of the stress and nature of the adaptive cellular response. How do the adaptive cellular responses decode such death programs that promote host-microbe survival leading to the establishment of chronic liver disease? In this review, we discuss how the adaptive cellular response through the Nrf2 pathway that promotes virus and cell survival. Furthermore, we provide a glimpse of novel stress-induced Nrf2 mediated compensatory autophagy mechanisms in virus-cell survival that degrade tumor suppressor gene and activation of oncogenic signaling during HCV infection. Based on these facts, we hypothesize that the balance between hepatic stress, inflammation and different types of cell death determines liver disease progression outcomes. We propose that a more nuanced understanding of virus-host interactions under excessive cellular stress may provide an answer to the fundamental questions why some individuals with chronic HCV infection remain at risk of developing cirrhosis, cancer and some do not.

Coupling caging and proteomics on the European flounder (Platichthys flesus) to assess the estuarine water quality at micro scale

Estuaries are important areas highly vulnerable to anthropogenic pollutions. Therefore, the assessment of estuarine water quality is a major ecological issue. In this study, we sampled juveniles of the European flounder in the "pristine" Canche estuary, and caged them in Canche and in two polluted sites of the Seine estuary, Rouen and Fosse Nord. After one month, the metal and organic pollutants in these sites were assessed, and we evaluated several phenotypic indicators (condition index, RNA/DNA ratios and genotoxicity), and extracted the proteins in fish livers for analysis using a shotgun proteomics approach. The results showed strong modifications in the fish caged in both sites of the Seine estuary, as compared to those caged in Canche. In particular, many proteins involved in phase I and phase II detoxification reactions were accumulated in the liver of fish caged in the site showing the highest pollution, Rouen. In addition, we observed a general disruption of metabolism, in particular an increase in lipid synthesis and carbohydrate degradation in Rouen, and a decrease in the abundance of proteins associated to translational activity in Fosse Nord. At both sites, several stress proteins were decreased. The proteomic impact of the encagement by itself was also evaluated, by comparing the liver proteome of fish caged in Canche to that of fish stayed in natura during the same time. The results showed proteomic signatures of exposure to stressful conditions (particularly heat stress), most probably related to the micro-habitat in which the cages were placed. In conclusion, the caging technique is of great interest for ecotoxicological assessment of estuarine waters, but should consider that the results are representative of the micro-habitat around the cages, which does not necessarily represent the overall heterogeneity of the estuarine environment.

Targeting the unfolded protein response in head and neck and oral cavity cancers

Many FDA-approved anti-cancer therapies, targeted toward a wide array of molecular targets and signaling networks, have been demonstrated to activate the unfolded protein response (UPR). Despite a critical role for UPR signaling in the apoptotic execution of cancer cells by many of these compounds, the authors are currently unaware of any instance whereby a cancer drug was developed with the UPR as the intended target. With the essential role of the UPR as a driving force in the genesis and maintenance of the malignant phenotype, a great number of pre-clinical studies have surged into the medical literature describing the ability of dozens of compounds to induce UPR signaling in a myriad of cancer models. The focus of the current work is to review the literature and explore the role of the UPR as a mediator of chemotherapy-induced cell death in squamous cell carcinomas of the head and neck (HNSCC) and oral cavity (OCSCC), with an emphasis on preclinical studies.

News from Mars: Two-Tier Paradox, Intracellular PCR, Chimeric Junction Shift, Dark Matter mRNA and Other Remarkable Features of Mammalian RNA-Dependent mRNA Amplification. Implications for Alzheimer's Disease, RNA-Based Vaccines and mRNA Therapeutics

Molecular Biology, a branch of science established to examine the flow of information from "letters" encrypted into DNA structure to functional proteins, was initially defined by a concept of DNA-to-RNA-to-Protein information movement, a notion termed the Central Dogma of Molecular Biology. RNA-dependent mRNA amplification, a novel mode of eukaryotic protein-encoding RNA-to-RNA-to-Protein genomic information transfer, constitutes the extension of the Central Dogma in the context of mammalian cells. It was shown to occur in cellular circumstances requiring exceptionally high levels of production of specific polypeptides, e.g. globin chains during erythroid differentiation or defined secreted proteins in the context of extracellular matrix deposition. Its potency is reflected in the observed cellular levels of the resulting amplified mRNA product: At the peak of the erythroid differentiation, for example, the amount of globin mRNA produced in the amplification pathway is about 1500-fold higher than the amount of its conventionally generated counterpart in the same cells. The cellular enzymatic machinery at the core of this process, RNA-dependent RNA polymerase activity (RdRp), albeit in a non-conventional form, was shown to be constitutively and ubiquitously present, and RNA-dependent RNA synthesis (RdRs) appeared to regularly occur, in mammalian cells. Under most circumstances, the mammalian RdRp activity produces only short antisense RNA transcripts. Generation of complete antisense RNA transcripts and amplification of mRNA molecules require the activation of inducible components of the mammalian RdRp complex. The mechanism of such activation is not clear. The present article suggests that it is triggered by a variety of cellular stresses and occurs in the context of stress responses in general and within the framework of the integrated stress response (ISR) in particular. In this process, various cellular stresses activate, in a stress type-specific manner, defined members of the mammalian translation initiation factor 2α, eIF2α, kinase family: PKR, GCN2, PERK and HRI. Any of these kinases, in an activated form, phosphorylates eIF2α. This results in suppression of global cellular protein synthesis but also in activation of expression of select group of transcription factors including ATF4, ATF5 and CHOP. These transcription factors either function as inducible components of the RdRp complex or enable their expression. The assembly of the competent RdRp complex activates mammalian RNA-dependent mRNA amplification, which appears to be a two-tier process. Tier One is a "chimeric" pathway, named so because it results in an amplified chimeric mRNA molecule containing a fragment of the antisense RNA strand at its 5' terminus. Tier Two further amplifies one of the two RNA end products of the chimeric pathway and constitutes the physiologically occurring intracellular polymerase chain reaction, iPCR. Depending on the structure of the initial mRNA amplification progenitor, the chimeric pathway, Tier One, may result in multiple outcomes including chimeric mRNA that produces either a polypeptide identical to the original, conventional mRNA progenitor-encoded protein or only its C-terminal fragment, CTF. The chimeric RNA end product of Tier One may also produce a polypeptide that is non-contiguously encoded in the genome, activate translation from an open reading frame, which is "silent" in a conventionally transcribed mRNA, or initiate an abortive translation. In sharp contrast, regardless of the outcome of Tier One, the mRNA end product of Tier Two of mammalian mRNA amplification, the iPCR pathway, always produces a polypeptide identical to a conventional mRNA progenitor-encoded protein. This discordance is referred to as the Two-Tier Paradox and discussed in detail in the present article. On the other hand, both Tiers are similar in that they result in heavily modified mRNA molecules resistant to reverse transcription, undetectable by reverse transcription-based methods of sequencing and therefore constituting a proverbial "Dark Matter" mRNA, despite being highly ubiquitous. It appears that in addition to their other functions, the modifications of the amplified mRNA render it compatible, unlike the bulk of cellular mRNA, with phosphorylated eIF2α in translation, implying that in addition to being extraordinarily abundant due to the method of its generation, amplified mRNA is also preferentially translated under the ISR conditions, thus augmenting the efficiency of the amplification process. The vital importance of powerful mechanisms of amplification of protein-encoding genomic information in normal physiology is self-evident. Their malfunctions or misuse appear to be associated with two types of abnormalities, the deficiency of a protein normally produced by these mechanisms and the mRNA amplification-mediated overproduction of a protein normally not generated by such a process. Certain classes of beta-thalassemia exemplify the first type, whereas the second type is represented by overproduction of beta-amyloid in Alzheimer's disease. Moreover, the proposed mechanism of Alzheimer's disease allows a crucial and verifiable prediction, namely that the disease-causing intraneuronally retained variant of beta-amyloid differs from that produced conventionally by βAPP proteolysis in that it contains the additional methionine or acetylated methionine at its N-terminus. Because of its extraordinary evidential value as a natural reporter of the mRNA amplification pathway, this feature, if proven, would, arguably, constitute the proverbial Holy Grail not only for Alzheimer's disease but also for the mammalian RNA-dependent mRNA amplification field in general. Both examples are discussed in detail in the present article, which summarizes and systematizes our current understanding of the field and describes two categories of reporter constructs, one for the chimeric Tier of mRNA amplification, another for the iPCR pathway; both reporter types are essential for elucidating underlying molecular mechanisms. It also suggests, in light of the recently demonstrated feasibility of RNA-based vaccines, that the targeted intracellular amplification of exogenously introduced amplification-eligible antigen-encoding mRNAs via the induced or naturally occurring RNA-dependent mRNA amplification pathway could be of substantial benefit in triggering a fast and potent immune response and instrumental in the development of future vaccines. Similar approaches can also be effective in achieving efficient and sustained expression of exogenous mRNA in mRNA therapeutics.

Human Host Range Restriction of the Vaccinia Virus C7/K1 Double Deletion Mutant Is Mediated by an Atypical Mode of Translation Inhibition

Replication of vaccinia virus in human cells depends on the viral C7 or K1 protein. A previous human genome-wide short interfering RNA (siRNA) screen with a C7/K1 double deletion mutant revealed SAMD9 as a principal host range restriction factor along with additional candidates, including WDR6 and FTSJ1. To compare their abilities to restrict replication, the cellular genes were individually inactivated by CRISPR/Cas9 mutagenesis. The C7/K1 deletion mutant exhibited enhanced replication in each knockout (KO) cell line but reached wild-type levels only in SAMD9 KO cells. SAMD9 was not depleted in either WDR6 or FTSJ1 KO cells, suggesting less efficient alternative rescue mechanisms. Using the SAMD9 KO cells as controls, we verified a specific block in host and viral intermediate and late protein synthesis in HeLa cells and demonstrated that the inhibition could be triggered by events preceding viral DNA replication. Inhibition of cap-dependent and -independent protein synthesis occurred primarily at the translational level, as supported by DNA and mRNA transfection experiments. Concurrent with collapse of polyribosomes, viral mRNA was predominantly in 80S and lighter ribonucleoprotein fractions. We confirmed the accumulation of cytoplasmic granules in HeLa cells infected with the C7/K1 deletion mutant and further showed that viral mRNA was sequestered with SAMD9. RNA granules were still detected in G3BP KO U2OS cells, which remained nonpermissive for the C7/K1 deletion mutant. Inhibition of cap-dependent and internal ribosome entry site-mediated translation, sequestration of viral mRNA, and failure of PKR, RNase L, or G3BP KO cells to restore protein synthesis support an unusual mechanism of host restriction.IMPORTANCE A dynamic relationship exists between viruses and their hosts in which each ostensibly attempts to exploit the other's vulnerabilities. A window is opened into the established condition, which evolved over millennia, if loss-of-function mutations occur in either the virus or host. Thus, the inability of viral host range mutants to replicate in specific cells can be overcome by identifying and inactivating the opposing cellular gene. Here, we investigated a C7/K1 host range mutant of vaccinia virus in which the cellular gene SAMD9 serves as the principal host restriction factor. Host restriction was triggered early in infection and manifested as a block in translation of viral mRNAs. Features of the block include inhibition of cap-dependent and internal ribosome entry site-mediated translation, sequestration of viral RNA, and inability to overcome the inhibition by inactivation of protein kinase R, ribonuclease L, or G3 binding proteins, suggesting a novel mechanism of host restriction.

A novel uORF-based regulatory mechanism controls translation of the human MDM2 and eIF2D mRNAs during stress

Short upstream open reading frames (uORFs) are the most prevalent cis-acting regulatory elements in the mammalian transcriptome which can orchestrate mRNA translation. Apart from being "passive roadblocks" that decrease expression of the main coding regions, particular uORFs can serve as specific sensors for changing conditions, thus regulating translation in response to cell stress. Here we report a novel uORF-based regulatory mechanism that is employed under conditions of hyperosmotic stress by at least two human mRNAs, coding for translation reinitiation/recycling factor eIF2D and E3 ubiquitin ligase MDM2. This novel mode of translational control selectively downregulates their expression and requires as few as one uORF. Using a set of reporter mRNAs and fleeting mRNA transfection (FLERT) technique, we provide evidence that the phenomenon does not rely on delayed reinitiation, altered AUG recognition, ribosome stalling, mRNA destabilization or other known mechanisms. Instead, it is based on events taking place at uORF stop codon or immediately downstream. Functional aspects and implications of the novel regulatory mechanism to cell physiology are discussed.

Inhibiting the integrated stress response pathway prevents aberrant chondrocyte differentiation thereby alleviating chondrodysplasia

The integrated stress response (ISR) is activated by diverse forms of cellular stress, including endoplasmic reticulum (ER) stress, and is associated with diseases. However, the molecular mechanism(s) whereby the ISR impacts on differentiation is incompletely understood. Here, we exploited a mouse model of Metaphyseal Chondrodysplasia type Schmid (MCDS) to provide insight into the impact of the ISR on cell fate. We show the protein kinase RNA-like ER kinase (PERK) pathway that mediates preferential synthesis of ATF4 and CHOP, dominates in causing dysplasia by reverting chondrocyte differentiation via ATF4-directed transactivation of Sox9. Chondrocyte survival is enabled, cell autonomously, by CHOP and dual CHOP-ATF4 transactivation of Fgf21. Treatment of mutant mice with a chemical inhibitor of PERK signaling prevents the differentiation defects and ameliorates chondrodysplasia. By preventing aberrant differentiation, titrated inhibition of the ISR emerges as a rationale therapeutic strategy for stress-induced skeletal disorders.

ROS signaling and ER stress in cardiovascular disease

The endoplasmic reticulum (ER) produces the vast majority of all proteins secreted into the extracellular space, including hormones and cytokines, as well as cell surface receptors and other proteins which interact with the environment. Accordingly, this organelle controls essentially all vital links to a cell's external milieu, responding to systemic metabolic, inflammatory, endocrine, and mechanical stimuli. The central role the ER plays in meeting protein synthetic and quality control requirements in the face of such demands is matched by an extensive and versatile ER stress response signaling network. ROS mediate several critical aspects of this response. Nox4, an ER resident capable of producing ROS, acts as a proximal signaling intermediate to transduce ER stress-related conditions to the unfolded protein response, a homeostatic corrective mechanism. However, chronic ER stress caused by unrelenting internal or external demands produces a secondary rise in ROS, generally resulting in cell death. Sorting out the involvement of ROS at different levels of the ER stress response in specific cell types is key to understanding the molecular basis for chronic diseases such as atherosclerosis, hypertension, and diabetes. Here, we provide an overview of ER stress signaling with an emphasis on the role of ROS.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

Borrelidin Induces the Unfolded Protein Response in Oral Cancer Cells and Chop-Dependent Apoptosis

Oral squamous cell carcinoma (OSCC) is the most common cancer affecting the oral cavity, and US clinics will register about 30,000 new patients in 2015. Current treatment modalities include chemotherapy, surgery, and radiotherapy, which often result in astonishing disfigurement. Cancers of the head and neck display enhanced levels of glucose-regulated proteins and translation initiation factors associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Previous work demonstrated that chemically enforced UPR could overwhelm these adaptive features and selectively kill malignant cells. The threonyl-tRNA synthetase (ThRS) inhibitor borrelidin and two congeners were discovered in a cell-based chemical genomic screen. Borrelidin increased XBP1 splicing and led to accumulation of phosphorylated eIF2α and UPR-associated genes, prior to death in panel of OSCC cells. Murine embryonic fibroblasts (MEFs) null for GCN2 and PERK were less able to accumulate UPR markers and were resistant to borrelidin. This study demonstrates that UPR induction is a feature of ThRS inhibition and adds to a growing body of literature suggesting ThRS inhibitors might selectively target cancer cells.

L protease from foot and mouth disease virus confers eIF2-independent translation for mRNAs bearing picornavirus IRES

The leader protease (L(pro)) from foot-and-mouth disease virus (FMDV) has the ability to cleave eIF4G, leading to a blockade of cellular protein synthesis. In contrast to previous reports, our present findings demonstrate that FMDV L(pro) is able to increase translation driven by FMDV IRES. Additionally, inactivation of eIF2 subsequent to phosphorylation induced by arsenite or thapsigargin in BHK cells blocks protein synthesis directed by FMDV IRES, whereas in the presence of L(pro), significant translation is found under these conditions. This phenomenon was also observed in cell-free systems after induction of eIF2 phosphorylation by addition of poly(I:C).

Phosphorylation of eIF2α is dispensable for differentiation but required at a posttranscriptional level for paneth cell function and intestinal homeostasis in mice

BACKGROUND

Recent studies link endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) to inflammatory bowel disease. Altered eIF2α phosphorylation (eIF2α-P), a regulatory hub of the UPR, was observed in mucosal tissue of patients with inflammatory bowel disease. In this study, we examined the mechanistic role of eIF2α-P in intestinal epithelial cell (IEC) function and intestinal homeostasis in mice.

METHODS

We generated mice with villin-Cre-mediated conditional expression of nonphosphorylatable Ser51Ala mutant eIF2α in IECs (AA mice). We analyzed AA mice under normal conditions and on challenge with oral infection of Salmonella Typhimurium or dextran sulfate sodium-induced colitis.

RESULTS

Loss of eIF2α-P did not affect the normal proliferation or differentiation of IECs. However, AA mice expressed decreased secretory proteins including lysozyme, suggesting eIF2α-P is required for Paneth cell function. The ultrastructure of AA Paneth cells exhibited a reduced number of secretory granules, a fragmented ER, and distended mitochondria under normal conditions. UPR gene expression was defective in AA IECs. Translation of Paneth cell specific messenger RNAs encoding lysozyme and cryptidins was significantly defective leading to the observed granule-deficient phenotype, which was associated with reduced ribosomal recruitment of these messenger RNAs to the ER membrane. Consequently, AA mice were more susceptible to oral Salmonella infection and dextran sulfate sodium-induced colitis.

CONCLUSIONS

We conclude eIF2α phosphorylation is required for the normal function of intestinal Paneth cells and mucosal homeostasis by activating UPR signaling and promoting messenger RNA recruitment to the ER membrane for translation.

Host cell factors as antiviral targets in arenavirus infection

Among the members of the Arenaviridae family, Lassa virus and Junin virus generate periodic annual outbreaks of severe human hemorrhagic fever (HF) in endemic areas of West Africa and Argentina, respectively. Given the human health threat that arenaviruses represent and the lack of a specific and safe chemotherapy, the search for effective antiviral compounds is a continuous demanding effort. Since diverse host cell pathways and enzymes are used by RNA viruses to fulfill their replicative cycle, the targeting of a host process has turned an attractive antiviral approach in the last years for many unrelated virus types. This strategy has the additional benefit to reduce the serious challenge for therapy of RNA viruses to escape from drug effects through selection of resistant variants triggered by their high mutation rate. This article focuses on novel strategies to identify inhibitors for arenavirus therapy, analyzing the potential for antiviral developments of diverse host factors essential for virus infection.

Lysophosphatidic acid differentially regulates axonal mRNA translation through 5'UTR elements

Sensory neurons transport a complex population of mRNAs into their axons, including many encoding ER chaperone proteins. Transport of the mRNA encoding the ER chaperone protein calreticulin is regulated through 3'UTR elements. In other cellular systems, translation of chaperone protein mRNAs can be regulated by ER stress. Here, we have asked if the translation of axonal calreticulin mRNA is regulated in a different manner than its transport into axons. Treatment with lysophosphatidic acid, which is known to trigger axon retraction and stimulate ER Ca(2+) release, caused a translation-dependent increase in axonal calreticulin protein levels. RNA sequences in the 5'UTR of calreticulin confer this translational control through a mechanism that requires an inactivating phosphorylation of eIF2α. In contrast to calreticulin, these signaling events do not activate axonal translation through β-actin's 5'UTR. Together, these data indicate that stimulation of ER stress can regulate specificity of localized mRNA translation through 5'UTR elements.

Sensitivity of global translation to mTOR inhibition in REN cells depends on the equilibrium between eIF4E and 4E-BP1

Initiation is the rate-limiting phase of protein synthesis, controlled by signaling pathways regulating the phosphorylation of translation factors. Initiation has three steps, 43S, 48S and 80S formation. 43S formation is repressed by eIF2α phosphorylation. The subsequent steps, 48S and 80S formation are enabled by growth factors. 48S relies on eIF4E-mediated assembly of eIF4F complex; 4E-BPs competitively displace eIF4E from eIF4F. Two pathways control eIF4F: 1) mTORc1 phosphorylates and inactivates 4E-BPs, leading to eIF4F formation; 2) the Ras-Mnk cascade phosphorylates eIF4E. We show that REN and NCI-H28 mesothelioma cells have constitutive activation of both pathways and maximal translation rate, in the absence of exogenous growth factors. Translation is rapidly abrogated by phosphorylation of eIF2α. Surprisingly, pharmacological inhibition of mTORc1 leads to the complete dephosphorylation of downstream targets, without changes in methionine incorporation. In addition, the combined administration of mTORc1 and MAPK/Mnk inhibitors has no additive effect. The inhibition of both mTORc1 and mTORc2 does not affect the metabolic rate. In spite of this, mTORc1 inhibition reduces eIF4F complex formation, and depresses translocation of TOP mRNAs on polysomes. Downregulation of eIF4E and overexpression of 4E-BP1 induce rapamycin sensitivity, suggesting that disruption of eIF4F complex, due to eIF4E modulation, competes with its recycling to ribosomes. These data suggest the existence of a dynamic equilibrium in which eIF4F is not essential for all mRNAs and is not displaced from translated mRNAs, before recycling to the next.

Junín virus infection impairs stress-granule formation in Vero cells treated with arsenite via inhibition of eIF2α phosphorylation

Stress granules (SGs) are ephemeral cytoplasmic aggregates containing stalled translation preinitiation complexes involved in mRNA storage and triage during the cellular stress response. SG formation is triggered by the phosphorylation of the alpha subunit of eIF2 (eIF2α), which provokes a dramatic blockage of protein translation. Our results demonstrate that acute infection of Vero cells with the arenavirus Junín (JUNV), aetiological agent of Argentine haemorrhagic fever, does not induce the formation of SGs. Moreover, JUNV negatively modulates SG formation in infected cells stressed with arsenite, and this inhibition correlates with low levels of eIF2α phosphorylation. Transient expression of JUNV nucleoprotein (N) or the glycoprotein precursor (GPC), but not of the matrix protein (Z), inhibits SG formation in a similar manner, comparable to infectious virus. Expression of N and GPC also impaired eIF2α phosphorylation triggered by arsenite. A moderate inhibition of SG formation was also observed when DTT and thapsigargin were employed as stress inducers. In contrast, no inhibition was observed when infected cells were treated with hippuristanol, a translational inhibitor and inducer of SGs that bypasses the requirement for eIF2α phosphorylation. Finally, we analysed SG formation in persistently JUNV-infected cells, where N and GPC are virtually absent and truncated N products are expressed abundantly. We found that persistently infected cells show a quite normal response to arsenite, with SG formation comparable to that of uninfected cells. This suggests that the presence of GPC and/or N is crucial to control the stress response upon JUNV infection of Vero cells.

Ca2+-signaling, alternative splicing and endoplasmic reticulum stress responses

Ca(2+)-signaling, alternative splicing, and stress responses by the endoplasmic reticulum are three important cellular activities which can be strongly interconnected to alter the expression of protein isoforms in a tissue dependent manner or during development depending on the environmental conditions. This integrated network of signaling pathways permits a high degree of versatility and adaptation to metabolic, developmental and stress processes. Defects in its regulation may lead to cellular malfunction.

The role of polyamines in supporting growth of mammalian cells is mediated through their requirement for translation initiation and elongation

Polyamines are essential cell constituents whose depletion results in growth cessation. Here we have investigated potential mechanisms of action of polyamines in supporting mammalian cell proliferation. We demonstrate that polyamines regulate translation both at the initiation and at the elongation steps. L-alpha-difluoromethylornithine treatment resulting in polyamine depletion reduces protein synthesis via inhibition of translation initiation. N1-guanyl-diaminoheptane (GC7), a spermidine analogue that inhibits eukaryotic initiation factor 5A (eIF5A) hypusination, also caused inhibition of translation initiation. In contrast, depletion of eIF5A by short hairpin RNA inhibits translation elongation as was recently demonstrated in yeast and Drosophila. These results suggest that in addition to competing with spermidine in the hypusination reaction, GC7 also competes with spermidine at yet undefined sites required for translation initiation. Finally, we show that either polyamine depletion or GC7 treatment induced eIF2alpha phosphorylation and reduced phosphorylation of 4E-BP, thus setting the molecular basis for the observed inhibition of translation initiation.

Oxidative folding: cellular strategies for dealing with the resultant equimolar production of reactive oxygen species

All eukaryotic cells possess an endoplasmic reticulum (ER), which is the site for synthesizing proteins that populate the cell surface or extracellular space. The environment of the ER is oxidizing, which supports the formation of intra- and interchain disulfide bonds that serve to stabilize the folding and assembly of nascent proteins. Recent experimental data reveal that the formation of disulfide bonds does not occur spontaneously but results from the enzymatic transfer of disulfide bonds through a number of intermediate proteins, with molecular oxygen serving as the terminal electron acceptor. Thus, each disulfide bond that forms during oxidative folding should produce a single reactive oxygen species (ROS). Dedicated secretory tissues like the pancreas and plasma cells have been estimated to form up to 3-6 million disulfide bonds per minute, which would be expected to result in the production of the same number of molecules of ROS. Although the methods used to deal with this amount of oxidative stress are not well understood, recent research suggests that different types of cells use distinct strategies and that the unfolded protein response (UPR) is a critical component of the defense.

The role of nitric-oxide synthase in the regulation of UVB light-induced phosphorylation of the alpha subunit of eukaryotic initiation factor 2

UV light induces phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2alpha) and inhibits global protein synthesis. Both eIF2 kinases, protein kinase-like endoplasmic reticulum kinase (PERK) and general control of nonderepressible protein kinase 2 (GCN2), have been shown to phosphorylate eIF2alpha in response to UV irradiation. However, the roles of PERK and GCN2 in UV-induced eIF2alpha phosphorylation are controversial. The one or more upstream signaling pathways that lead to the activation of PERK or GCN2 remain unknown. In this report we provide data showing that both PERK and GCN2 contribute to UV-induced eIF2alpha phosphorylation in human keratinocyte (HaCaT) and mouse embryonic fibroblast cells. Reduction of expression of PERK or GCN2 by small interfering RNA decreases phosphorylation of eIF2alpha after UV irradiation. These data also show that nitric-oxide synthase (NOS)-mediated oxidative stress plays a role in regulation of eIF2alpha phosphorylation upon UV irradiation. Treating the cells with the broad NOS inhibitor N(G)-methyl-l-arginine, the free radical scavenger N-acetyl-l-cysteine, or the NOS substrate l-arginine partially inhibits UV-induced eIF2alpha phosphorylation. The results presented above led us to propose that NOS mediates UV-induced eIF2alpha phosphorylation by activation of both PERK and GCN2 via oxidative stress and l-arginine starvation signaling pathways.

Regulation of mRNA transport and translation in axons

Movement of mRNAs into axons occurs by active transport by microtubules through the activity of molecular motor proteins. mRNAs are sequestered into granular-like particles, referred to as transport ribonucleoprotein particles (RNPs) that mediate transport into the axonal compartment. The interaction of mRNA binding proteins with targeted mRNA is a key event in regulating axonal mRNA localization and subsequent localized translation of mRNAs. Several growth-modulating stimuli have been shown to regulate axonal mRNA localization. These do so by activating specific intracellular signaling pathways that converge upon RNA binding proteins and other components of the transport RNP to regulate their activity specifically. Transport can be both positively and negatively regulated by individual stimuli with regard to individual mRNAs. Consequently, there is exquisite specificity for regulating the axon's composition of mRNAs and proteins that control expression in the axon. Finally, recent studies indicate that axotomy can also trigger changes in axonal mRNA composition by specifically shifting the populations of mRNAs that are transported into distal axons.

Essential role of PACT-mediated PKR activation in tunicamycin-induced apoptosis

Cellular stresses such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and lead to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double-stranded RNA (dsRNA)-activated protein kinase (PKR) and its protein activator PACT in tunicamycin-induced apoptosis. We demonstrate for the first time that PACT is phosphorylated in response to tunicamycin and is responsible for PKR activation by direct interaction. Furthermore, PACT-induced PKR activation is essential for tunicamycin-induced apoptosis, since PACT as well as PKR null cells are markedly resistant to tunicamycin and show defective eIF2alpha phosphorylation and C/EBP homologous protein (CHOP, also known as GADD153) induction especially at low concentrations of tunicamycin. Reconstitution of PKR and PACT expression in the null cells renders them sensitive to tunicamycin, thus demonstrating that PACT-induced PKR activation plays an essential function in induction of apoptosis.

Fragile X mental retardation protein shifts between polyribosomes and stress granules after neuronal injury by arsenite stress or in vivo hippocampal electrode insertion

Fragile X mental retardation protein (FMRP), the lack of which causes fragile X syndrome, is an RNA-binding protein encoded by the FMR1 gene. FMRP accompanies mRNAs from the nucleus to dendritic regions and is thought to regulate their translation at synapses. It has been shown that FMRP moves into nontranslating stress granules (SGs) during heat stress of cultured fibroblasts (Mazroui et al., 2002). We used a novel method to isolate SGs from neurons by virtue of their TIA-1 (T-cell intracellular antigen 1) protein component, and found that FMRP moved out of polyribosomes and into SGs subsequent to oxidative stress. We then examined FMRP changes in subcellular localization resulting from mechanically induced neuronal injury by placement of electrodes into the dentate gyrus and the perforant path of the hippocampus in vivo. During the first 10 min after electrode insertion into one hippocampus, FMRP shifted into SGs and away from polyribosomes, in both hippocampi. Although the injury discharge subsided beyond 10 s, FMRP levels in polyribosomes and stress granules did not return to basal levels until 30 min after electrode penetration. Our findings suggest that procedures for in vivo induction of long-term potentiation or long-term depression should incorporate a 30 min rest period after electrode insertion, and indicate that the contralateral hippocampus cannot be considered an unstimulated control tissue.