Targeting stress granules: A novel therapeutic strategy for human diseases

G3BP1/2-Targeting PROTAC Disrupts Stress Granules Dependent ATF4 Migracytosis as Cancer Therapy

Stress granules (SGs) are membraneless cytoplasmic compartments that form in response to stress stimuli. In these compartments, most translation factors stall, except for activating transcription factor 4 (ATF4), which is preferentially translated to ensure cell survival under stressful conditions. Cancer cells encounter various stress conditions in the tumor microenvironment during tumorigenesis; however, how they exploit the pro-survival effects of ATF4 in SGs remains unclear. G3BP1/2 are central nodes of the SG network, regulating SG dynamics. In this study, we designed two small molecules, #129 and PROTAC (Proteolysis Targeting Chimera) degrader 129 (PT-129), which specifically target the NTF2L domain of G3BP1/2, a crucial hub for protein and RNA interactions. These compounds inhibit the formation of stress granules in stressed cells and disassemble pre-existing stress granules. Furthermore, pharmacological inhibition by PT-129 suppressed fibroblast-mediated cancer cell growth in vitro and reduced tumor growth in vivo. Mechanistically, SG facilitates the delivery of ATF4 from fibroblasts to tumor cells via migracytosis, a primary mediator of fibroblast-associated tumor growth. PT-129-mediated disassembly of stress granules disrupts ATF4 delivery, thereby preventing cancer cell proliferation. These compounds, therefore, represent powerful tools for gaining molecular insights into SGs and hold promise for cancer therapeutic interventions by modulating stress granule dynamics.

Stress granules play a critical role in hexavalent chromium-induced malignancy in a G3BP1 dependent manner

Stress granules (SGs) are dynamic membraneless organelles influencing multiple cellular pathways including cell survival, proliferation, and malignancy. Hexavalent chromium [Cr(VI)] is a toxic heavy metal associated with severe environmental health risks. Low-level environmental exposure to Cr(VI) has been reported to cause cancer, but the role of SGs in Cr(VI)-induced health effects remains unclear. This study was intended to elucidate the impact of Cr(VI) exposure on SG dynamics and the role of SGs in Cr(VI)-induced malignancy. Results showed that both acute exposure to high concentration of Cr(VI) and prolonged exposure to low concentration of Cr(VI)-induced SG formation in human bronchial epithelium BEAS-2B cells. Cells pre-exposed to Cr(VI) exhibited a more robust SG response compared to cells without pre-exposure. An up-regulated SG response was associated with increased malignant properties in cells exposed to low concentration Cr(VI) for an extended period of time up to 12 months. Knocking out the SG core protein G3BP1 in Cr(VI)-transformed (CrT) cells reduced SG formation and malignant properties, including proliferation rate, sphere formation, and malignant markers. The results support a critical role for SGs in mediating Cr(VI)-induced malignancy in a G3BP1-dependent manner, representing a novel mechanism and a potential therapeutic target.

Calcium signaling from damaged lysosomes induces cytoprotective stress granules

Lysosomal damage induces stress granule (SG) formation. However, the importance of SGs in determining cell fate and the precise mechanisms that mediate SG formation in response to lysosomal damage remain unclear. Here, we describe a novel calcium-dependent pathway controlling SG formation, which promotes cell survival during lysosomal damage. Mechanistically, the calcium-activated protein ALIX transduces lysosomal damage signals to SG formation by controlling eIF2α phosphorylation after sensing calcium leakage. ALIX enhances eIF2α phosphorylation by promoting the association between PKR and its activator PACT, with galectin-3 inhibiting this interaction; these regulatory events occur on damaged lysosomes. We further find that SG formation plays a crucial role in promoting cell survival upon lysosomal damage caused by factors such as SARS-CoV-2ORF3a, adenovirus, malarial pigment, proteopathic tau, or environmental hazards. Collectively, these data provide insights into the mechanism of SG formation upon lysosomal damage and implicate it in diseases associated with damaged lysosomes and SGs.

SARS-CoV-2 Nucleocapsid Protein Antagonizes GADD34-Mediated Innate Immune Pathway through Atypical Foci

The integrated stress response, especially stress granules (SGs), contributes to host immunity. Typical G3BP1+ stress granules (tSGs) are usually formed after virus infection to restrain viral replication and stimulate innate immunity. Recently, several SG-like foci or atypical SGs (aSGs) with proviral function have been found during viral infection. We have shown that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein induces atypical N+/G3BP1+ foci (N+foci), leading to the inhibition of host immunity and facilitation of viral infection. However, the precise mechanism has not been well clarified yet. In this study, we showed that the SARS-CoV-2 N (SARS2-N) protein inhibits dsRNA-induced growth arrest and DNA damage-inducible 34 (GADD34) expression. Mechanistically, the SARS2-N protein promotes the interaction between GADD34 mRNA and G3BP1, sequestering GADD34 mRNA into the N+foci. Importantly, we found that GADD34 participates in IRF3 nuclear translocation through its KVRF motif and promotes the transcription of downstream interferon genes. The suppression of GADD34 expression by the SARS2-N protein impairs the nuclear localization of IRF3 and compromises the host's innate immune response, which facilitates viral replication. Taking these findings together, our study revealed a novel mechanism by which the SARS2-N protein antagonized the GADD34-mediated innate immune pathway via induction of N+foci. We think this is a critical strategy for viral pathogenesis and has potential therapeutic implications.

MAD-microbial (origin of) Alzheimer's disease hypothesis: from infection and the antimicrobial response to disruption of key copper-based systems

Microbes have been suspected to cause Alzheimer's disease since at least 1908, but this has generally remained unpopular in comparison to the amyloid hypothesis and the dominance of Aβ and Tau. However, evidence has been accumulating to suggest that these earlier theories are but a manifestation of a common cause that can trigger and interact with all the major molecular players recognized in AD. Aβ, Tau and ApoE, in particular appear to be molecules with normal homeostatic functions but also with alternative antimicrobial functions. Their alternative functions confer the non-immune specialized neuron with some innate intracellular defenses that appear to be re-appropriated from their normal functions in times of need. Indeed, signs of infection of the neurons by biofilm-forming microbial colonies, in synergy with herpes viruses, are evident from the clinical and preclinical studies we discuss. Furthermore, we attempt to provide a mechanistic understanding of the AD landscape by discussing the antimicrobial effect of Aβ, Tau and ApoE and Lactoferrin in AD, and a possible mechanistic link with deficiency of vital copper-based systems. In particular, we focus on mitochondrial oxidative respiration via complex 4 and ceruloplasmin for iron homeostasis, and how this is similar and possibly central to neurodegenerative diseases in general. In the case of AD, we provide evidence for the microbial Alzheimer's disease (MAD) theory, namely that AD could in fact be caused by a long-term microbial exposure or even long-term infection of the neurons themselves that results in a costly prolonged antimicrobial response that disrupts copper-based systems that govern neurotransmission, iron homeostasis and respiration. Finally, we discuss potential treatment modalities based on this holistic understanding of AD that incorporates the many separate and seemingly conflicting theories. If the MAD theory is correct, then the reduction of microbial exposure through use of broad antimicrobial and anti-inflammatory treatments could potentially alleviate AD although this requires further clinical investigation.

Stress granules in atherosclerosis: Insights and therapeutic opportunities

Atherosclerosis, a complex inflammatory and metabolic disorder, is the underlying cause of several life-threatening cardiovascular diseases. Stress granules (SG) are biomolecular condensates composed of proteins and mRNA that form in response to stress. Recent studies suggest a potential link between SG and atherosclerosis development. However, there remain gaps in understanding SG role in atherosclerosis development. Here we provide a thorough analysis of the role of SG in atherosclerosis, covering cellular stresses stimulation, core components, and regulatory genes in SG formation. Furthermore, we explore atherosclerosis induced factors such as inflammation, low or oscillatory shear stress (OSS), and oxidative stress (OS) may impact SG formation and then the development of atherosclerotic lesions. We have assessed how changes in SG dynamics impact pro-atherogenic processes like endothelial dysfunction, lipid metabolism, and immune cell recruitment in atherosclerosis. In summary, this review emphasizes the complex interplay between SG and atherosclerosis that could open innovative directions for targeted therapeutic strategies in preventing or treating atherosclerotic cardiovascular diseases.

Proteasome resides in and dismantles plant heat stress granules constitutively

Stress granules (SGs) are conserved reversible cytoplasmic condensates enriched with aggregation-prone proteins assembled in response to various stresses. How plants regulate SG dynamics is unclear. Here, we show that 26S proteasome is a stable component of SGs, promoting the overall clearance of SGs without affecting the molecular mobility of SG components. Increase in either temperature or duration of heat stress reduces the molecular mobility of SG marker proteins and suppresses SG clearance. Heat stress induces dramatic ubiquitylation of SG components and enhances the activities of SG-resident proteasomes, allowing the degradation of SG components even during the assembly phase. Their proteolytic activities enable the timely disassembly of SGs and secure the survival of plant cells during the recovery from heat stress. Therefore, our findings identify the cellular process that de-couples macroscopic dynamics of SGs from the molecular dynamics of its constituents and highlights the significance of the proteasomes in SG disassembly.

Liquid-liquid phase separation in diseases

Liquid-liquid phase separation (LLPS), an emerging biophysical phenomenon, can sequester molecules to implement physiological and pathological functions. LLPS implements the assembly of numerous membraneless chambers, including stress granules and P-bodies, containing RNA and protein. RNA-RNA and RNA-protein interactions play a critical role in LLPS. Scaffolding proteins, through multivalent interactions and external factors, support protein-RNA interaction networks to form condensates involved in a variety of diseases, particularly neurodegenerative diseases and cancer. Modulating LLPS phenomenon in multiple pathogenic proteins for the treatment of neurodegenerative diseases and cancer could present a promising direction, though recent advances in this area are limited. Here, we summarize in detail the complexity of LLPS in constructing signaling pathways and highlight the role of LLPS in neurodegenerative diseases and cancers. We also explore RNA modifications on LLPS to alter diseases progression because these modifications can influence LLPS of certain proteins or the formation of stress granules, and discuss the possibility of proper manipulation of LLPS process to restore cellular homeostasis or develop therapeutic drugs for the eradication of diseases. This review attempts to discuss potential therapeutic opportunities by elaborating on the connection between LLPS, RNA modification, and their roles in diseases.

Establishment of a stress granule reporter system for evaluating in vitro colon toxicity

Exposure to toxic molecules from food or oral medications induces toxicity in colon cells that cause various human diseases; however, in vitro monitoring systems for colon cell toxicity are not well established. Stress granules are nonmembranous foci that form in cells exposed to cellular stress. When cells sense toxic environments, they acutely and systemically promote stress granule formation, with Ras GTPase-activating protein-binding protein 1 (G3BP1) acting as a core component to protect their mRNA from abnormal degradation. Here, we knocked in green fluorescent protein (GFP)-coding sequences into the C-terminal region of the G3BP1 gene in a human colon cell line through CRISPR-Cas9-mediated homologous recombination and confirmed the formation of stress granules with the G3BP1-GFP protein in these cells under cellular stress exposure. We demonstrated the formation and dissociation of stress granules in G3BP1-GFP expressing colon cells through real-time monitoring using a fluorescence microscope. Furthermore, we validated the toxicity monitoring system in the established colon cell line by observing stress granule formation following exposure to dihydrocapsaicin, bisphenol A, and sorbitol. Taken together, we established a stress granule reporter system in a colon cell line, providing a novel assessment for the real-time monitoring of colon toxicity in response to various chemicals.

Formation of the NLRP3 inflammasome inhibits stress granule assembly by multiple mechanisms

Proper regulation of cellular response to environmental stress is crucial for maintaining biological homeostasis and is achieved by the balance between cell death processes, such as the formation of the pyroptosis-inducing NLRP3 inflammasome, and pro-survival processes, such as stress granule (SG) assembly. However, the functional interplay between these two stress-responsive organelles remains elusive. Here, we identified DHX33, a viral RNA sensor for the NLRP3 inflammasome, as a SG component, and the SG-nucleating protein G3BP as an NLRP3 inflammasome component. We also found that a decrease in intracellular potassium (K+) concentration, a key 'common' step in NLRP3 inflammasome activation, markedly inhibited SG assembly. Therefore, when macrophages are exposed to stress stimuli with the potential to induce both SGs and the NLRP3 inflammasome, such as cytoplasmic poly(I:C) stimulation, they preferentially form the NLRP3 inflammasome but avoid SG assembly by sequestering G3BP into the inflammasome and by inducing a reduction in intracellular K+ levels. Thus, under such conditions, DHX33 is primarily utilized as a viral RNA sensor for the inflammasome. Our data reveal the functional crosstalk between NLRP3 inflammasome-mediated pyroptosis and SG-mediated cell survival pathways and delineate a molecular mechanism that regulates cell-fate decisions and anti-viral innate immunity under stress.

Stress Granule Core Protein-Derived Peptides Inhibit Assembly of Stress Granules and Improve Sorafenib Sensitivity in Cancer Cells

Upon a variety of environmental stresses, eukaryotic cells usually recruit translational stalled mRNAs and RNA-binding proteins to form cytoplasmic condensates known as stress granules (SGs), which minimize stress-induced damage and promote stress adaptation and cell survival. SGs are hijacked by cancer cells to promote cell survival and are consequently involved in the development of anticancer drug resistance. However, the design and application of chemical compounds targeting SGs to improve anticancer drug efficacy have rarely been studied. Here, we developed two types of SG inhibitory peptides (SIPs) derived from SG core proteins Caprin1 and USP10 and fused with cell-penetrating peptides to generate TAT-SIP-C1/2 and SIP-U1-Antp, respectively. We obtained 11 SG-inducing anticancer compounds from cell-based screens and explored the potential application of SIPs in overcoming resistance to the SG-inducing anticancer drug sorafenib. We found that SIPs increased the sensitivity of HeLa cells to sorafenib via the disruption of SGs. Therefore, anticancer drugs which are competent to induce SGs could be combined with SIPs to sensitize cancer cells, which might provide a novel therapeutic strategy to alleviate anticancer drug resistance.

Targeting stress granules in neurodegenerative diseases: A focus on biological function and dynamics disorders

Stress granules (SGs) are membraneless organelles formed by eukaryotic cells in response to stress to promote cell survival through their pleiotropic cytoprotective effects. SGs recruit a variety of components to enhance their physiological function, and play a critical role in the propagation of pathological proteins, a key factor in neurodegeneration. Recent advances indicate that SG dynamic disorders exacerbate neuronal susceptibility to stress in neurodegenerative diseases (NDs) including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Huntington's disease (HD) and Parkinson's disease (PD). Here, we outline the biological functions of SGs, highlight SG dynamic disorders in NDs, and emphasize therapeutic approaches for enhancing SG dynamics to provide new insights into ND intervention.

Structured and disordered regions of Ataxin-2 contribute differently to the specificity and efficiency of mRNP granule formation

Ataxin-2 (ATXN2) is a gene implicated in spinocerebellar ataxia type II (SCA2), amyotrophic lateral sclerosis (ALS) and Parkinsonism. The encoded protein is a therapeutic target for ALS and related conditions. ATXN2 (or Atx2 in insects) can function in translational activation, translational repression, mRNA stability and in the assembly of mRNP-granules, a process mediated by intrinsically disordered regions (IDRs). Previous work has shown that the LSm (Like-Sm) domain of Atx2, which can help stimulate mRNA translation, antagonizes mRNP-granule assembly. Here we advance these findings through a series of experiments on Drosophila and human Ataxin-2 proteins. Results of Targets of RNA Binding Proteins Identified by Editing (TRIBE), co-localization and immunoprecipitation experiments indicate that a polyA-binding protein (PABP) interacting, PAM2 motif of Ataxin-2 may be a major determinant of the mRNA and protein content of Ataxin-2 mRNP granules. Experiments with transgenic Drosophila indicate that while the Atx2-LSm domain may protect against neurodegeneration, structured PAM2- and unstructured IDR- interactions both support Atx2-induced cytotoxicity. Taken together, the data lead to a proposal for how Ataxin-2 interactions are remodelled during translational control and how structured and non-structured interactions contribute differently to the specificity and efficiency of RNP granule condensation as well as to neurodegeneration.

A mechanism that transduces lysosomal damage signals to stress granule formation for cell survival

Lysosomal damage poses a significant threat to cell survival. Our previous work has reported that lysosomal damage induces stress granule (SG) formation. However, the importance of SG formation in determining cell fate and the precise mechanisms through which lysosomal damage triggers SG formation remains unclear. Here, we show that SG formation is initiated via a novel calcium-dependent pathway and plays a protective role in promoting cell survival in response to lysosomal damage. Mechanistically, we demonstrate that during lysosomal damage, ALIX, a calcium-activated protein, transduces lysosomal damage signals by sensing calcium leakage to induce SG formation by controlling the phosphorylation of eIF2α. ALIX modulates eIF2α phosphorylation by regulating the association between PKR and its activator PACT, with galectin-3 exerting a negative effect on this process. We also found this regulatory event of SG formation occur on damaged lysosomes. Collectively, these investigations reveal novel insights into the precise regulation of SG formation triggered by lysosomal damage, and shed light on the interaction between damaged lysosomes and SGs. Importantly, SG formation is significant for promoting cell survival in the physiological context of lysosomal damage inflicted by SARS-CoV-2 ORF3a, adenovirus infection, Malaria hemozoin, proteopathic tau as well as environmental hazard silica.

DDX3X and Stress Granules: Emerging Players in Cancer and Drug Resistance

The DEAD (Asp-Glu-Ala-Asp)-box helicase 3 X-linked (DDX3X) protein participates in many aspects of mRNA metabolism and stress granule (SG) formation. DDX3X has also been associated with signal transduction and cell cycle regulation that are important in maintaining cellular homeostasis. Malfunctions of DDX3X have been implicated in multiple cancers, including brain cancer, leukemia, prostate cancer, and head and neck cancer. Recently, literature has reported SG-associated cancer drug resistance, which correlates with a negative disease prognosis. Based on the connections between DDX3X, SG formation, and cancer pathology, targeting DDX3X may be a promising direction for cancer therapeutics development. In this review, we describe the biological functions of DDX3X in terms of mRNA metabolism, signal transduction, and cell cycle regulation. Furthermore, we summarize the contributions of DDX3X in SG formation and cellular stress adaptation. Finally, we discuss the relationships of DDX3X, SG, and cancer drug resistance, and discuss the current research progress of several DDX3X inhibitors for cancer treatment.

The SARS-CoV-2 nucleocapsid protein suppresses innate immunity by remodeling stress granules to atypical foci

Viruses deploy multiple strategies to suppress the host innate immune response to facilitate viral replication and pathogenesis. Typical G3BP1+ stress granules (SGs) are usually formed in host cells after virus infection to restrain viral translation and to stimulate innate immunity. Thus, viruses have evolved various mechanisms to inhibit SGs or to repurpose SG components such as G3BP1. Previous studies showed that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection inhibited host immunity during the early stage of COVID-19. However, the precise mechanism is not yet well understood. Here we showed that the SARS-CoV-2 nucleocapsid (SARS2-N) protein suppressed the double-stranded RNA (dsRNA)-induced innate immune response, concomitant with inhibition of SGs and the induction of atypical SARS2-N+ /G3BP1+ foci (N+ foci). The SARS2-N protein-induced formation of N+ foci was dependent on the ability of its ITFG motif to hijack G3BP1, which contributed to suppress the innate immune response. Importantly, SARS2-N protein facilitated viral replication by inducing the formation of N+ foci. Viral mutations within SARS2-N protein that impair the formation of N+ foci are associated with the inability of the SARS2-N protein to suppress the immune response. Taken together, our study has revealed a novel mechanism by which SARS-CoV-2 suppresses the innate immune response via induction of atypical N+ foci. We think that this is a critical strategy for viral pathogenesis and has potential therapeutic implications.

Volumetric segmentation of biological cells and subcellular structures for optical diffraction tomography images

Three-dimensional, quantitative imaging of biological cells and their internal structures performed by optical diffraction tomography (ODT) is an important part of biomedical research. However, conducting quantitative analysis of ODT images requires performing 3D segmentation with high accuracy, often unattainable with available segmentation methods. Therefore, in this work, we present a new semi-automatic method, called ODT-SAS, which combines several non-machine-learning techniques to segment cells and 2 types of their organelles: nucleoli and lipid structures (LS). ODT-SAS has been compared with Cellpose and slice-by-slice manual segmentation, respectively, in cell segmentation and organelles segmentation. The comparison shows superiority of ODT-SAS over Cellpose and reveals the potential of our technique in detecting cells, nucleoli and LS.

Chloromethylisothiazolinone induces ER stress-induced stress granule formation in human keratinocytes

Chloromethylisothiazolinone (CMIT), a humidifier disinfectant, is known to be toxic to the respiratory system. While the toxic effect of CMIT on the lungs has been widely investigated, its effect on the skin is well unknown. In this study, we examined stress granule (SG) formation to investigate the cytotoxic effects of CMIT on human keratinocytes. We assessed the viability of the cells following CMIT exposure and performed immunofluorescence microscopy and immunoblot analyses to determine SG formation and downstream pathways. The IC50 values in human keratinocyte HaCaT cells after CMIT exposure for 1 and 24 h were 11 and 8 μg/mL, respectively, showing no significant difference. As determined using immunofluorescence microscopy, SG formation was effectively induced after CMIT exposure. Moreover, the phosphorylation of eukaryotic initiation factor-2α (eIF2α), a translation initiation factor, and protein kinase R-like endoplasmic reticulum (ER) kinase, which plays a role in the ER stress-mediated eIF2α phosphorylation, was confirmed by CMIT exposure. These results suggest that exposure to CMIT can have detrimental effects on the skin, even briefly, by inducing SG formation through ER stress in keratinocytes.

C9orf72 gene networks in the human brain correlate with cortical thickness in C9-FTD and implicate vulnerable cell types

Introduction

A hexanucleotide repeat expansion (HRE) intronic to chromosome 9 open reading frame 72 (C9orf72) is recognized as the most common genetic cause of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and ALS-FTD. Identifying genes that show similar regional co-expression patterns to C9orf72 may help identify novel gene targets and biological mechanisms that mediate selective vulnerability to ALS and FTD pathogenesis.

Methods

We leveraged mRNA expression data in healthy brain from the Allen Human Brain Atlas to evaluate C9orf72 co-expression patterns. To do this, we correlated average C9orf72 expression values in 51 regions across different anatomical divisions (cortex, subcortex, cerebellum) with average gene expression values for 15,633 protein-coding genes, including 50 genes known to be associated with ALS, FTD, or ALS-FTD. We then evaluated whether the identified C9orf72 co-expressed genes correlated with patterns of cortical thickness in symptomatic C9orf72 pathogenic HRE carriers (n=19). Lastly, we explored whether genes with significant C9orf72 radiogenomic correlations (i.e., 'C9orf72 gene network') were enriched in specific cell populations in the brain and enriched for specific biological and molecular pathways.

Results

A total of 1,748 genes showed an anatomical distribution of gene expression in the brain similar to C9orf72 and significantly correlated with patterns of cortical thickness in C9orf72 HRE carriers. This C9orf72 gene network was differentially expressed in cell populations previously implicated in ALS and FTD, including layer 5b cells, cholinergic motor neurons in the spinal cord, and medium spiny neurons of the striatum, and was enriched for biological and molecular pathways associated with multiple neurotransmitter systems, protein ubiquitination, autophagy, and MAPK signaling, among others.

Conclusions

Considered together, we identified a network of C9orf72-associated genes that may influence selective regional and cell-type-specific vulnerabilities in ALS/FTD.

Involvement of small molecules and metabolites in regulation of biomolecular condensate properties

Biomolecular condensate (BMCs) formation facilitates the grouping of molecules, including proteins, nucleic acids, and small molecules, creating specific microenvironments with particular functions. They are often assembled through liquid-liquid phase separation (LLPS), a phenomenon that arises when specific proteins, nucleic acids, and small molecules demix from the aqueous environment into another phase with different physiochemical properties. BMCs assemble and disassemble in response to external and internal stimuli such as temperature, molecule concentration, ionic strength, pH, and cellular redox state. Likewise, the nature of the regulatory stimuli may affect the lifespan, morphology, and content of BMCs. In humans, compelling evidence points to the critical role of BMCs in diseases. By contrast, the link between BMC formation, stress resistance, and cell survival has not been revealed in plants. Recent studies have pointed out the nascent roles of small molecules in the assembly and dynamics of BMCs; however, this is still an emerging field of study. This review briefly highlights the most significant efforts to identify the molecular mechanisms between small molecules and BMC formation and regulation in plants and other organisms. We then discuss (i) how small molecules exert control over the BMC assembly and dynamics in plants and (ii) how small molecules can influence the formation and material properties of plant BMCs. Finally, we propose novel alternatives that might help to understand the relationship between chemicals and condensation dynamics and their possible application to plant biotechnology.

Therapeutic potential of natural compounds from herbs and nutraceuticals in spinal cord injury: Regulation of the mTOR signaling pathway

Spinal cord injury (SCI) is a disease in which the spinal cord is subjected to various external forces that cause it to burst, shift, or, in severe cases, injure the spinal tissue, resulting in nerve injury. SCI includes not only acute primary injury but also delayed and persistent spinal tissue injury (i.e., secondary injury). The pathological changes post-SCI are complex, and effective clinical treatment strategies are lacking. The mammalian target of rapamycin (mTOR) coordinates the growth and metabolism of eukaryotic cells in response to various nutrients and growth factors. The mTOR signaling pathway has multiple roles in the pathogenesis of SCI. There is evidence for the beneficial effects of natural compounds and nutraceuticals that regulate the mTOR signaling pathways in a variety of diseases. Therefore, the effects of natural compounds on the pathogenesis of SCI were evaluated by a comprehensive review using electronic databases, such as PubMed, Web of Science, Scopus, and Medline, combined with our expertise in neuropathology. In particular, we reviewed the pathogenesis of SCI, including the importance of secondary nerve injury after the primary mechanical injury, the roles of the mTOR signaling pathways, and the beneficial effects and mechanisms of natural compounds that regulate the mTOR signaling pathway on pathological changes post-SCI, including effects on inflammation, neuronal apoptosis, autophagy, nerve regeneration, and other pathways. This recent research highlights the value of natural compounds in regulating the mTOR pathway, providing a basis for developing novel therapeutic strategies for SCI.

Stress granules affect the sensitivity of renal cancer cells to sorafenib by sequestering and stabilizing COX‑2 mRNA

Most patients with renal cancer will develop resistance to sorafenib therapy and will therefore exhibit disease progression. Effective therapies for these patients are extremely limited. Cyclooxygenase-2 (COX-2) promotes the malignant transformation of cancer cells and drug resistance. The potential of COX-2 inhibitor (celecoxib) administration in combination with sorafenib for the treatment of renal cancer is unclear. The present study demonstrated that sorafenib rapidly increased the expression of COX-2 in renal cancer cells, as determined using reverse transcription-quantitative PCR and western blotting. The results of the MTT assay and cell apoptosis experiment demonstrated that the cytotoxicity of sorafenib was also affected by COX-2 expression and celecoxib enhanced the cytotoxicity of sorafenib against renal cell carcinoma. Immunofluorescence analysis indicated that sorafenib induced the formation of stress granules (SGs) in renal cancer cells. In addition, COX-2 expression was associated with the formation of SGs, and SGs could capture and stabilize COX-2 mRNAs in renal cancer cells; this was confirmed using RNA fluorescence in situ hybridization and an actinomycin D chase experiment. The protective effect of SGs was further demonstrated in cell experiments and xenograft tumor models. Thus, the present study indicated that the use of celecoxib may significantly enhance the sensitivity of renal cancer cells to sorafenib and improve efficacy. Sorafenib-induced SGs may contribute to critical events that promote COX-2 expression and survival in renal cancer cells. Therefore, the present study may provide novel ideas for the treatment of renal cancer.

Stress granule formation inhibits stress-induced apoptosis by selectively sequestering executioner caspases

Cytoplasmic stress granules (SGs) are phase-separated membrane-less organelles that form in response to various stress stimuli. SGs are mainly composed of non-canonical stalled 48S preinitiation complexes. In addition, many other proteins also accumulate into SGs, but the list is still incomplete. SG assembly suppresses apoptosis and promotes cell survival under stress. Furthermore, hyperformation of SGs is frequently observed in various human cancers and accelerates tumor development and progression by reducing stress-induced damage of cancer cells. Therefore, they are of clinical importance. However, the precise mechanism underlying SG-mediated inhibition of apoptosis remains ill-defined. Here, using a proximity-labeling proteomic approach, we comprehensively analyzed SG-resident proteins and identified the executioner caspases, caspase-3 and -7, as SG components. We demonstrate that accumulation of caspase-3/7 into SGs is mediated by evolutionarily conserved amino acid residues within their large catalytic domains and inhibits caspase activities and consequent apoptosis induced by various stresses. Expression of an SG-localization-deficient caspase-3 mutant in cells largely counteracted the anti-apoptotic effect of SGs, whereas enforced relocalization of the caspase-3 mutant to SGs restored it. Thus, SG-mediated sequestration of executioner caspases is a mechanism underlying the broad cytoprotective function of SGs. Furthermore, using a mouse xenograft tumor model, we show that this mechanism prevents cancer cells from apoptosis in tumor tissues, thereby promoting cancer progression. Our results reveal the functional crosstalk between SG-mediated cell survival and caspase-mediated cell death signaling pathways and delineate a molecular mechanism that dictates cell-fate decisions under stress and promotes tumorigenesis.

Development of FRET and Stress Granule Dual-Based System to Screen for Viral 3C Protease Inhibitors

3C proteases (3Cpros) of picornaviruses and 3C-like proteases (3CLpros) of coronaviruses and caliciviruses represent a group of structurally and functionally related viral proteases that play pleiotropic roles in supporting the viral life cycle and subverting host antiviral responses. The design and screening for 3C/3CLpro inhibitors may contribute to the development broad-spectrum antiviral therapeutics against viral diseases related to these three families. However, current screening strategies cannot simultaneously assess a compound’s cytotoxicity and its impact on enzymatic activity and protease-mediated physiological processes. The viral induction of stress granules (SGs) in host cells acts as an important antiviral stress response by blocking viral translation and stimulating the host immune response. Most of these viruses have evolved 3C/3CLpro-mediated cleavage of SG core protein G3BP1 to counteract SG formation and disrupt the host defense. Yet, there are no SG-based strategies screening for 3C/3CLpro inhibitors. Here, we developed a fluorescence resonance energy transfer (FRET) and SG dual-based system to screen for 3C/3CLpro inhibitors in living cells. We took advantage of FRET to evaluate the protease activity of poliovirus (PV) 3Cpro and live-monitor cellular SG dynamics to cross-verify its effect on the host antiviral response. Our drug screen uncovered a novel role of Telaprevir and Trifluridine as inhibitors of PV 3Cpro. Moreover, Telaprevir and Trifluridine also modulated 3Cpro-mediated physiological processes, including the cleavage of host proteins, inhibition of the innate immune response, and consequent facilitation of viral replication. Taken together, the FRET and SG dual-based system exhibits a promising potential in the screening for inhibitors of viral proteases that cleave G3BP1.

Multiple functions of stress granules in viral infection at a glance

Stress granules (SGs) are distinct RNA granules induced by various stresses, which are evolutionarily conserved across species. In general, SGs act as a conservative and essential self-protection mechanism during stress responses. Viruses have a long evolutionary history and viral infections can trigger a series of cellular stress responses, which may interact with SG formation. Targeting SGs is believed as one of the critical and conservative measures for viruses to tackle the inhibition of host cells. In this systematic review, we have summarized the role of SGs in viral infection and categorized their relationships into three tables, with a particular focus on Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection. Moreover, we have outlined several kinds of drugs targeting SGs according to different pathways, most of which are potentially effective against SARS-CoV-2. We believe this review would offer a new view for the researchers and clinicians to attempt to develop more efficacious treatments for virus infection, particularly for the treatment of SARS-CoV-2 infection.

How do RNA binding proteins trigger liquid-liquid phase separation in human health and diseases?

RNA-binding proteins (RBPs) lie at the center of post-transcriptional regulation and protein synthesis, adding complexity to RNA life cycle. RBPs also participate in the formation of membrane-less organelles (MLOs) via undergoing liquid-liquid phase separation (LLPS), which underlies the formation of MLOs in eukaryotic cells. RBPs-triggered LLPS mainly relies on the interaction between their RNA recognition motifs (RRMs) and capped mRNA transcripts and the heterotypic multivalent interactions between their intrinsically disordered regions (IDRs) or prion-like domains (PLDs). In turn, the aggregations of RBPs are also dependent on the process of LLPS. RBPs-driven LLPS is involved in many intracellular processes (regulation of translation, mRNA storage and stabilization and cell signaling) and serves as the heart of cellular physiology and pathology. Thus, it is essential to comprehend the potential roles and investigate the internal mechanism of RPBs-triggered LLPS. In this review, we primarily expound on our current understanding of RBPs and they-triggered LLPS and summarize their physiological and pathological functions. Furthermore, we also summarize the potential roles of RBPs-triggered LLPS as novel therapeutic mechanism for human diseases. This review will help understand the mechanisms underlying LLPS and downstream regulation of RBPs and provide insights into the pathogenesis and therapy of complex diseases.

Liquid-liquid phase separation in tumor biology

Liquid–liquid phase separation (LLPS) is a novel principle for explaining the precise spatial and temporal regulation in living cells. LLPS compartmentalizes proteins and nucleic acids into micron-scale, liquid-like, membraneless bodies with specific functions, which were recently termed biomolecular condensates. Biomolecular condensates are executors underlying the intracellular spatiotemporal coordination of various biological activities, including chromatin organization, genomic stability, DNA damage response and repair, transcription, and signal transduction. Dysregulation of these cellular processes is a key event in the initiation and/or evolution of cancer, and emerging evidence has linked the formation and regulation of LLPS to malignant transformations in tumor biology. In this review, we comprehensively summarize the detailed mechanisms of biomolecular condensate formation and biophysical function and review the recent major advances toward elucidating the multiple mechanisms involved in cancer cell pathology driven by aberrant LLPS. In addition, we discuss the therapeutic perspectives of LLPS in cancer research and the most recently developed drug candidates targeting LLPS modulation that can be used to combat tumorigenesis.

Glucocorticoids enhance chemotherapy-driven stress granule assembly and impair granule dynamics leading to cell death

Stress granules (SGs) can assemble in cancer cells upon chemotoxic stress. Glucocorticoids function during stress responses and are administered with chemotherapies. The roles of glucocorticoids in SG assembly and disassembly pathways are unknown. We examined whether combining glucocorticoids such as cortisone with chemotherapies from the vinca alkaloid family, which dismantle the microtubule network, affects SG assembly and disassembly pathways and influences cell viability in cancer cells and human-derived organoids. Cortisone augmented SG formation when combined with vinorelbine (VRB). Live-cell imaging showed that cortisone increased SG assembly rates but reduced SG clearance rates after stress, by increasing protein residence times within the SGs. Mechanistically, VRB and cortisone signaled through the integrated stress response mediated by eIF2α (also known as EIF2S1), yet induced different kinases, with cortisone activating the GCN2 kinase (also known as EIF2AK4). Cortisone increased VRB-induced cell death and reduced the population of cells trapped in mitotic catastrophe. These effects were mediated by the core SG proteins G3BP1 and G3BP2. In conclusion, glucocorticoids induce SG assembly and cell death when administered with chemotherapies, suggesting that combining glucocorticoids with chemotherapies can enhance cancer cell chemosensitivity.

Summary: Combining cortisone with the chemotherapy vinorelbine enhances the assembly of stress granules that are less likely to be cleared from the cells, augmenting vinorelbine-induced cell death.

Systematic Analysis of Stress Granule Regulators-Associated Molecular Subtypes Predicts Drug Response, Immune Response, and Prognosis in Non-Small Cell Lung Cancer

Lung cancer has the world’s second highest cancer incidence and second highest cancer-related mortality rate. However, the mechanism underlying non-small cell lung cancer (NSCLC) remained to be unclear. Overall, this study for the first time revealed Stress Granule Regulators were mutated and dysregulated in NSCLC samples by analyzing TCGA database. Moreover, three subtypes of NSCLC were identified based on the expression levels of Stress Granule Regulators. Patients in cluster 2 showed a higher survival rate than those in clusters 1 and 3. Bioinformatics analysis indicated the cell cycle, mTOR signaling pathway, EGFR signaling, PI3K/Akt signaling and DNA damage repair signaling were significantly related to molecular subtypes. Moreover, we performed a prediction analysis of the response to the inhibitors against the aforementioned signaling. Our results showed patients in C2 NSCLC had the highest sensitivity to MK.2206 (AKT.inhibitor) and Rapamycin (mTOR inhibitor). Patients in C3 NSCLC had the highest sensitivity for Temsirolimus (PI3K/mTOR signaling), BIBW2992 (EGFR signaling), Erlotinib (EGFR signaling), PD.0332991 (CDK4/6 inhibitor), CGP.60474 (CDK inhibitor), and Gefitinib (EGFR signaling). Moreover, our results showed patients in C1 NSCLC had the highest sensitivity to AKT.inhibitor, AZD6482 (PI3K inhibitor). To evaluate the response to immune therapy of different subtypes, we analyzed the tumor immune inflation, immune regulators expression, and TIDE score in different SG related subtypes. These results showed that C2 and C3 may be more sensitive to immune therapy. To better predict the prognosis of NSCLC, we analyzed the correlation between stress granule regulator expression and overall survival time in NSCLC and constructed a Stress Granule Score including EIF2S1, CTSG, EIF4G1, IGF2BP1, PABPC1 to predict the prognosis of NSCLC. Overall, this study for the first time uncovers the effect of stress particles on drug response, immune response, and prognosis, laying a new theoretical foundation for the NSCLC prognosis and treatment.

Precision analysis of mutant U2AF1 activity reveals deployment of stress granules in myeloid malignancies

Splicing factor mutations are common among cancers, recently emerging as drivers of myeloid malignancies. U2AF1 carries hotspot mutations in its RNA-binding motifs; however, how they affect splicing and promote cancer remain unclear. The U2AF1/U2AF2 heterodimer is critical for 3' splice site (3'SS) definition. To specifically unmask changes in U2AF1 function in vivo, we developed a crosslinking and immunoprecipitation procedure that detects contacts between U2AF1 and the 3'SS AG at single-nucleotide resolution. Our data reveal that the U2AF1 S34F and Q157R mutants establish new 3'SS contacts at -3 and +1 nucleotides, respectively. These effects compromise U2AF2-RNA interactions, resulting predominantly in intron retention and exon exclusion. Integrating RNA binding, splicing, and turnover data, we predicted that U2AF1 mutations directly affect stress granule components, which was corroborated by single-cell RNA-seq. Remarkably, U2AF1-mutant cell lines and patient-derived MDS/AML blasts displayed a heightened stress granule response, pointing to a novel role for biomolecular condensates in adaptive oncogenic strategies.

Evasion of Host Antiviral Innate Immunity by Paramyxovirus Accessory Proteins

For efficient replication, viruses have developed multiple strategies to evade host antiviral innate immunity. Paramyxoviruses are a large family of enveloped RNA viruses that comprises diverse human and animal pathogens which jeopardize global public health and the economy. The accessory proteins expressed from the P gene by RNA editing or overlapping open reading frames (ORFs) are major viral immune evasion factors antagonizing type I interferon (IFN-I) production and other antiviral innate immune responses. However, the antagonistic mechanisms against antiviral innate immunity by accessory proteins differ among viruses. Here, we summarize the current understandings of immune evasion mechanisms by paramyxovirus accessory proteins, specifically how accessory proteins directly or indirectly target the adaptors in the antiviral innate immune signaling pathway to facilitate virus replication. Additionally, some cellular responses, which are also involved in viral replication, will be briefly summarized.

SARS-CoV-2 NSP5 and N protein counteract the RIG-I signaling pathway by suppressing the formation of stress granules

As a highly pathogenic human coronavirus, SARS-CoV-2 has to counteract an intricate network of antiviral host responses to establish infection and spread. The nucleic acid-induced stress response is an essential component of antiviral defense and is closely related to antiviral innate immunity. However, whether SARS-CoV-2 regulates the stress response pathway to achieve immune evasion remains elusive. In this study, SARS-CoV-2 NSP5 and N protein were found to attenuate antiviral stress granule (avSG) formation. Moreover, NSP5 and N suppressed IFN expression induced by infection of Sendai virus or transfection of a synthetic mimic of dsRNA, poly (I:C), inhibiting TBK1 and IRF3 phosphorylation, and restraining the nuclear translocalization of IRF3. Furthermore, HEK293T cells with ectopic expression of NSP5 or N protein were less resistant to vesicular stomatitis virus infection. Mechanistically, NSP5 suppressed avSG formation and disrupted RIG-I-MAVS complex to attenuate the RIG-I-mediated antiviral immunity. In contrast to the multiple targets of NSP5, the N protein specifically targeted cofactors upstream of RIG-I. The N protein interacted with G3BP1 to prevent avSG formation and to keep the cofactors G3BP1 and PACT from activating RIG-I. Additionally, the N protein also affected the recognition of dsRNA by RIG-I. This study revealed the intimate correlation between SARS-CoV-2, the stress response, and innate antiviral immunity, shedding light on the pathogenic mechanism of COVID-19.

SARS-CoV-2 nucleocapsid protein binds host mRNAs and attenuates stress granules to impair host stress response

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein is essential for viral replication, making it a promising target for antiviral drug and vaccine development. SARS-CoV-2 infected patients exhibit an uncoordinated immune response; however, the underlying mechanistic details of this imbalance remain obscure. Here, starting from a functional proteomics workflow, we cataloged the protein-protein interactions of SARS-CoV-2 proteins, including an evolutionarily conserved specific interaction of N with the stress granule resident proteins G3BP1 and G3BP2. N localizes to stress granules and sequesters G3BPs away from their typical interaction partners, thus attenuating stress granule formation. We found that N binds directly to host mRNAs in cells, with a preference for 3' UTRs, and modulates target mRNA stability. We show that the N protein rewires the G3BP1 mRNA-binding profile and suppresses the physiological stress response of host cells, which may explain the imbalanced immune response observed in SARS-CoV-2 infected patients.

Image-Based Screening for Stress Granule Regulators

Stress granule (SG)-based RNA interference (RNAi) screening is a powerful method to discover factors that control protein synthesis and aggregation, as well as regulators of SG assembly and disassembly. Here, we describe how to set up and optimize a large-scale siRNA screen, and give a detailed outline for the automated quantification of SGs as a visual readout. Hit evaluation via calculated Z scores provides a list of candidates for further in-depth studies.

Integrated stress response restricts macrophage necroptosis

Stress inhibits necroptosis in a PERK-dependent manner via reduced RIPK1-RIPK3-MLKL signaling, showing an integral mechanistic connection between stress responses and programmed cell death.

The integrated stress response (ISR) regulates cellular homeostasis and cell survival following exposure to stressors. Cell death processes such as apoptosis and pyroptosis are known to be modulated by stress responses, but the role of the ISR in necroptosis is poorly understood. Necroptosis is an inflammatory, lytic form of cell death driven by the RIPK3-MLKL signaling axis. Here, we show that macrophages that have induced the ISR are protected from subsequent necroptosis. Consistent with a reduction in necroptosis, phosphorylation of RIPK1, RIPK3, and MLKL is reduced in macrophages pre-treated with ISR-inducing agents that are challenged with necroptosis-inducing triggers. The stress granule component DDX3X, which is involved in ISR-mediated regulation of pyroptosis, is not required for protecting ISR-treated cells from necroptosis. Disruption of stress granule assembly or knockdown of Perk restored necroptosis in pre-stressed cells. Together, these findings identify a critical role for the ISR in limiting necroptosis in macrophages.

Attenuation of Activated eIF2α Signaling by ISRIB Treatment After Spinal Cord Injury Improves Locomotor Function

Following spinal cord injury (SCI), multiple signaling cascades are activated instantaneously in the injured segments of the spinal cord to create a complex and pathogenic microenvironment, making it difficult to treat SCI. Nevertheless, the significance of the integrated stress response (ISR) to the series of physiological and pathological changes that occur after SCI remains unclear. Through western blotting (WB), we determined that the autophosphorylation of stress receptors (GCN2, PERK, PKR, and HRI) was enhanced after SCI, leading to increased phosphorylation of eIF2α at Ser51. Strikingly, we found that eIF2α was highly phosphorylated at 1 day post injury (dpi) and that this hypophosphorylation was maintained thereafter in the spinal cord, especially in neurons, which suggests that intervening with eIF2α phosphorylation may be a treatment strategy for SCI. Therefore, we employed the small molecule ISRIB, which inhibits eIF2α phosphorylation when the ISR is activated at moderate or low levels but not when the ISR is highly activated. Daily intraperitoneal injection of ISRIB significantly inhibited ISR signaling after SCI, reduced the cytosolic localization of RNA-binding proteins, and decreased neuronal apoptosis. Histological and functional experiments further demonstrated that treatment with ISRIB after SCI effectively curbed morphological deterioration and promoted the recovery of locomotor function. In summary, the ISR plays an important role in SCI, and ISRIB is a promising drug for the treatment of SCI.

MG53 suppresses tumor progression and stress granule formation by modulating G3BP2 activity in non-small cell lung cancer

Background

Cancer cells develop resistance to chemotherapeutic intervention by excessive formation of stress granules (SGs), which are modulated by an oncogenic protein G3BP2. Selective control of G3BP2/SG signaling is a potential means to treat non-small cell lung cancer (NSCLC).

Methods

Co-immunoprecipitation was conducted to identify the interaction of MG53 and G3BP2. Immunohistochemistry and live cell imaging were performed to visualize the subcellular expression or co-localization. We used shRNA to knock-down the expression MG53 or G3BP2 to test the cell migration and colony formation. The expression level of MG53 and G3BP2 in human NSCLC tissues was tested by western blot analysis. The ATO-induced oxidative stress model was used to examine the effect of rhMG53 on SG formation. Moue NSCLC allograft experiments were performed on wild type and transgenic mice with either knockout of MG53, or overexpression of MG53. Human NSCLC xenograft model in mice was used to evaluate the effect of MG53 overexpression on tumorigenesis.

Results

We show that MG53, a member of the TRIM protein family (TRIM72), modulates G3BP2 activity to control lung cancer progression. Loss of MG53 results in the progressive development of lung cancer in mg53^-/- mice. Transgenic mice with sustained elevation of MG53 in the bloodstream demonstrate reduced tumor growth following allograft transplantation of mouse NSCLC cells. Biochemical assay reveals physical interaction between G3BP2 and MG53 through the TRIM domain of MG53. Knockdown of MG53 enhances proliferation and migration of NSCLC cells, whereas reduced tumorigenicity is seen in NSCLC cells with knockdown of G3BP2 expression. The recombinant human MG53 (rhMG53) protein can enter the NSCLC cells to induce nuclear translation of G3BP2 and block arsenic trioxide-induced SG formation. The anti-proliferative effect of rhMG53 on NSCLC cells was abolished with knockout of G3BP2. rhMG53 can enhance sensitivity of NSCLC cells to undergo cell death upon treatment with cisplatin. Tailored induction of MG53 expression in NSCLC cells suppresses lung cancer growth via reduced SG formation in a xenograft model.

Conclusion

Overall, these findings support the notion that MG53 functions as a tumor suppressor by targeting G3BP2/SG activity in NSCLCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01418-3.

Pesticides DEET, fipronil and maneb induce stress granule assembly and translation arrest in neuronal cells

Pesticides entering our body, either directly or indirectly, are known to increase the risk of developing neurodegenerative disorders. The pesticide-induced animal models of Parkinson's disease and Alzheimer's disease recapitulates many of the pathologies seen in human patients and have become popular models for studying disease biology. However, the specific effect of pesticides at the cellular and molecular levels is yet to be fully established. Here we investigated the cellular effect of three commonly used pesticides: DEET, fipronil and maneb. Specifically, we looked at the effect of these pesticides in the formation of stress granules and the concomitant translational arrest in a neuronal cell line. Stress granules represent an ensemble of non-translating mRNAs and appear in cells under physiological stress. Growing evidence indicates that chronic stress may covert the transient stress granules into amyloids and may thus induce neurodegeneration. We demonstrate here that all three pesticides tested induce stress granules and translation arrest through the inactivation of the eukaryotic initiation factor, eIF2α. We also show that oxidative stress could be one of the major intermediary factors in the pesticide-induced stress granule formation and that it is a reversible process. Our results suggest that prolonged pesticide exposure may result in long-lived stress granules, thus compromising the neuronal stress response pathway and leading to neurodegeneration.

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Pesticides exposure increases the risk of developing neurodegenerative disorders.

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Cellular effect of three common pesticides (DEET, fipronil and maneb) were tested.

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All three induce stress granules and translation arrest via inactivation of eIF2α.

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Compromised stress response may result in pesticide-induced neurodegeneration.

Stress granules safeguard against MAPK signaling hyperactivation by sequestering PKC/Pck2: new findings and perspectives

Stress granule (SG) assembly is a conserved cellular strategy that copes with stress-related damage and promotes cell survival. SGs form through a process of liquid-liquid phase separation. Cellular signaling also appears to employ SG assembly as a mechanism for controlling cell survival and cell death by spatial compartmentalization of signal-transducing factors. While several lines of evidence highlight the importance of SGs as signaling hubs, where protein components of signaling pathways can be temporarily sequestered, shielded from the cytoplasm, the regulation and physiological significance of SGs in this aspect remain largely obscure. A recent study of the heat-shock response in the fission yeast Schizosaaccharomyces pombe provides an unexpected answer to this question. Recently, we demonstrated that the PKC orthologue Pck2 in fission yeast translocates into SGs through phase separation in a PKC kinase activity-dependent manner upon high-heat stress (HHS). Importantly, the downstream MAPK Pmk1 promotes Pck2 recruitment into SGs, which intercepts MAPK hyperactivation and cell death, thus posing SGs as a negative feedback circuit in controlling MAPK signaling. Intriguingly, HHS, but not modest-heat stress targets Pck2 to SGs, independent of canonical SG machinery. Finally, cells fail to activate MAPK signaling when Pck2 is sequestrated into SGs. In this review, we will discuss how SGs have a role as signaling hubs beyond serving as a repository for non-translated mRNAs during acute stress.

Noncanonical Functions and Cellular Dynamics of the Mammalian Signal Recognition Particle Components

The signal recognition particle (SRP) is a ribonucleoprotein complex fundamental for co-translational delivery of proteins to their proper membrane localization and secretory pathways. Literature of the past two decades has suggested new roles for individual SRP components, 7SL RNA and proteins SRP9, SRP14, SRP19, SRP54, SRP68 and SRP72, outside the SRP cycle. These noncanonical functions interconnect SRP with a multitude of cellular and molecular pathways, including virus-host interactions, stress response, transcriptional regulation and modulation of apoptosis in autoimmune diseases. Uncovered novel properties of the SRP components present a new perspective for the mammalian SRP as a biological modulator of multiple cellular processes. As a consequence of these findings, SRP components have been correlated with a growing list of diseases, such as cancer progression, myopathies and bone marrow genetic diseases, suggesting a potential for development of SRP-target therapies of each individual component. For the first time, here we present the current knowledge on the SRP noncanonical functions and raise the need of a deeper understanding of the molecular interactions between SRP and accessory cellular components. We examine diseases associated with SRP components and discuss the development and feasibility of therapeutics targeting individual SRP noncanonical functions.

The Integral Role of RNA in Stress Granule Formation and Function

Stress granules (SGs) are phase-separated, membraneless, cytoplasmic ribonucleoprotein (RNP) assemblies whose primary function is to promote cell survival by condensing translationally stalled mRNAs, ribosomal components, translation initiation factors, and RNA-binding proteins (RBPs). While the protein composition and the function of proteins in the compartmentalization and the dynamics of assembly and disassembly of SGs has been a matter of study for several years, the role of RNA in these structures had remained largely unknown. RNA species are, however, not passive members of RNA granules in that RNA by itself can form homo and heterotypic interactions with other RNA molecules leading to phase separation and nucleation of RNA granules. RNA can also function as molecular scaffolds recruiting multivalent RBPs and their interactors to form higher-order structures. With the development of SG purification techniques coupled to RNA-seq, the transcriptomic landscape of SGs is becoming increasingly understood, revealing the enormous potential of RNA to guide the assembly and disassembly of these transient organelles. SGs are not only formed under acute stress conditions but also in response to different diseases such as viral infections, cancer, and neurodegeneration. Importantly, these granules are increasingly being recognized as potential precursors of pathological aggregates in neurodegenerative diseases. In this review, we examine the current evidence in support of RNA playing a significant role in the formation of SGs and explore the concept of SGs as therapeutic targets.

Hiding in Plain Sight: Formation and Function of Stress Granules During Microbial Infection of Mammalian Cells

Stress granule (SG) formation is a host cell response to stress-induced translational repression. SGs assemble with RNA-binding proteins and translationally silent mRNA. SGs have been demonstrated to be both inhibitory to viruses, as well as being subverted for viral roles. In contrast, the function of SGs during non-viral microbial infections remains largely unexplored. A handful of microbial infections have been shown to result in host SG assembly. Nevertheless, a large body of evidence suggests SG formation in hosts is a widespread response to microbial infection. Diverse stresses caused by microbes and their products can activate the integrated stress response in order to inhibit translation initiation through phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). This translational response in other contexts results in SG assembly, suggesting that SG assembly can be a general phenomenon during microbial infection. This review explores evidence for host SG formation in response to bacterial, fungal, and protozoan infection and potential functions of SGs in the host and for adaptations of the pathogen.

Dr. Jekyll and Mr. Hyde? Physiology and Pathology of Neuronal Stress Granules

Stress granules (SGs) are membraneless cytosolic granules containing dense aggregations of RNA-binding proteins and RNAs. They appear in the cytosol under stress conditions and inhibit the initiation of mRNA translation. SGs are dynamically assembled under stressful conditions and rapidly disassembled after stress removal. They are heterogeneous in their RNA and protein content and are cell type- and stress-specific. In post-mitotic neurons, which do not divide, the dynamics of neuronal SGs are tightly regulated, implying that their dysregulation leads to neurodegeneration. Mutations in RNA-binding proteins are associated with SGs. SG components accumulate in cytosolic inclusions in many neurodegenerative diseases, such as frontotemporal dementia and amyotrophic lateral sclerosis. Although SGs primarily mediate a pro-survival adaptive response to cellular stress, abnormal persistent SGs might develop into aggregates and link to the pathogenesis of diseases. In this review, we present recent advances in the study of neuronal SGs in physiology and pathology, and discuss potential therapeutic approaches to remove abnormal, persistent SGs associated with neurodegeneration.