The eIF2α kinase HRI triggers the autophagic clearance of cytosolic protein aggregates

Cytosolic retention of HtrA2 during mitochondrial protein import stress triggers the DELE1-HRI pathway

Mitochondrial stress inducers such as carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and oligomycin trigger the DELE1-HRI branch of the integrated stress response (ISR) pathway. Previous studies performed using epitope-tagged DELE1 showed that these stresses induced the cleavage of DELE1 to DELE1-S, which stimulates HRI. Here, we report that mitochondrial protein import stress (MPIS) is an overarching stress that triggers the DELE1-HRI pathway, and that endogenous DELE1 could be cleaved into two forms, DELE1-S and DELE1-VS, the latter accumulating only upon non-depolarizing MPIS. Surprisingly, while the mitochondrial protease OMA1 was crucial for DELE1 cleavage in HeLa cells, it was dispensable in HEK293T cells, suggesting that multiple proteases may be involved in DELE1 cleavage. In support, we identified a role for the mitochondrial protease, HtrA2, in mediating DELE1 cleavage into DELE1-VS, and showed that a Parkinson's disease (PD)-associated HtrA2 mutant displayed reduced DELE1 processing ability, suggesting a novel mechanism linking PD pathogenesis to mitochondrial stress. Our data further suggest that DELE1 is likely cleaved into DELE1-S in the cytosol, while the DELE1-VS form might be generated during halted translocation into mitochondria. Together, this study identifies MPIS as the overarching stress detected by DELE1 and identifies a novel role for HtrA2 in DELE1 processing.

ATF4 Signaling in HIV-1 Infection: Viral Subversion of a Stress Response Transcription Factor

Cellular integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt), and IFN signaling are associated with viral infections. Activating transcription factor 4 (ATF4) plays a pivotal role in these pathways and controls the expression of many genes involved in redox processes, amino acid metabolism, protein misfolding, autophagy, and apoptosis. The precise role of ATF4 during viral infection is unclear and depends on cell hosts, viral agents, and models. Furthermore, ATF4 signaling can be hijacked by pathogens to favor viral infection and replication. In this review, we summarize the ATF4-mediated signaling pathways in response to viral infections, focusing on human immunodeficiency virus 1 (HIV-1). We examine the consequences of ATF4 activation for HIV-1 replication and reactivation. The role of ATF4 in autophagy and apoptosis is explored as in the context of HIV-1 infection programmed cell deaths contribute to the depletion of CD4 T cells. Furthermore, ATF4 can also participate in the establishment of innate and adaptive immunity that is essential for the host to control viral infections. We finally discuss the putative role of the ATF4 paralogue, named ATF5, in HIV-1 infection. This review underlines the role of ATF4 at the crossroads of multiple processes reflecting host-pathogen interactions.

Cellular Stress Responses against Coronavirus Infection: A Means of the Innate Antiviral Defense

Cellular stress responses are crucial for maintaining cellular homeostasis. Stress granules (SGs), activated by eIF2α kinases in response to various stimuli, play a pivotal role in dealing with diverse stress conditions. Viral infection, as one kind of cellular stress, triggers specific cellular programs aimed at overcoming virus-induced stresses. Recent studies have revealed that virus-derived stress responses are tightly linked to the host's antiviral innate immunity. Virus infection-induced SGs act as platforms for antiviral sensors, facilitating the initiation of protective antiviral responses called "antiviral stress granules" (avSGs). However, many viruses, including coronaviruses, have evolved strategies to suppress avSG formation, thereby counteracting the host's immune responses. This review discusses the intricate relationship between cellular stress responses and antiviral innate immunity, with a specific focus on coronaviruses. Furthermore, the diverse mechanisms employed by viruses to counteract avSGs are described.

Signaling plasticity in the integrated stress response

The Integrated Stress Response (ISR) is an essential homeostatic signaling network that controls the cell's biosynthetic capacity. Four ISR sensor kinases detect multiple stressors and relay this information to downstream effectors by phosphorylating a common node: the alpha subunit of the eukaryotic initiation factor eIF2. As a result, general protein synthesis is repressed while select transcripts are preferentially translated, thus remodeling the proteome and transcriptome. Mounting evidence supports a view of the ISR as a dynamic signaling network with multiple modulators and feedback regulatory features that vary across cell and tissue types. Here, we discuss updated views on ISR sensor kinase mechanisms, how the subcellular localization of ISR components impacts signaling, and highlight ISR signaling differences across cells and tissues. Finally, we consider crosstalk between the ISR and other signaling pathways as a determinant of cell health.

The integrated stress response in cancer progression: a force for plasticity and resistance

During their quest for growth, adaptation, and survival, cancer cells create a favorable environment through the manipulation of normal cellular mechanisms. They increase anabolic processes, including protein synthesis, to facilitate uncontrolled proliferation and deplete the tumor microenvironment of resources. As a dynamic adaptation to the self-imposed oncogenic stress, cancer cells promptly hijack translational control to alter gene expression. Rewiring the cellular proteome shifts the phenotypic balance between growth and adaptation to promote therapeutic resistance and cancer cell survival. The integrated stress response (ISR) is a key translational program activated by oncogenic stress that is utilized to fine-tune protein synthesis and adjust to environmental barriers. Here, we focus on the role of ISR signaling for driving cancer progression. We highlight mechanisms of regulation for distinct mRNA translation downstream of the ISR, expand on oncogenic signaling utilizing the ISR in response to environmental stresses, and pinpoint the impact this has for cancer cell plasticity during resistance to therapy. There is an ongoing need for innovative drug targets in cancer treatment, and modulating ISR activity may provide a unique avenue for clinical benefit.

HSPB8 frameshift mutant aggregates weaken chaperone-assisted selective autophagy in neuromyopathies

Chaperone-assisted selective autophagy (CASA) is a highly selective pathway for the disposal of misfolding and aggregating proteins. In muscle, CASA assures muscle integrity by favoring the turnover of structural components damaged by mechanical strain. In neurons, CASA promotes the removal of aggregating substrates. A crucial player of CASA is HSPB8 (heat shock protein family B (small) member 8), which acts in a complex with HSPA, their cochaperone BAG3, and the E3 ubiquitin ligase STUB1. Recently, four novel HSPB8 frameshift (fs) gene mutations have been linked to neuromyopathies, and encode carboxy-terminally mutated HSPB8, sharing a common C-terminal extension. Here, we analyzed the biochemical and functional alterations associated with the HSPB8_fs mutant proteins. We demonstrated that HSPB8_fs mutants are highly insoluble and tend to form proteinaceous aggregates in the cytoplasm. Notably, all HSPB8 frameshift mutants retain their ability to interact with CASA members but sequester them into the HSPB8-positive aggregates together with two autophagy receptors SQSTM1/p62 and TAX1BP1. This copartitioning process negatively affects the CASA capability to remove its clients and causes a general failure in proteostasis response. Further analyses revealed that the aggregation of the HSPB8_fs mutants occurs independently of the other CASA members or from the autophagy receptors interaction, but it is an intrinsic feature of the mutated amino acid sequence. HSPB8_fs mutants aggregation alters the differentiation capacity of muscle cells and impairs sarcomere organization. Collectively, these results shed light on a potential pathogenic mechanism shared by the HSPB8_fs mutants described in neuromuscular diseases.Abbreviations : ACD: α-crystallin domain; ACTN: actinin alpha; BAG3: BAG cochaperone 3; C: carboxy; CASA: chaperone-assisted selective autophagy; CE: carboxy-terminal extension; CLEM: correlative light and electron microscopy; CMT2L: Charcot-Marie-Tooth type 2L; CTR: carboxy-terminal region; dHMNII: distal hereditary motor neuropathy type II; EV: empty vector; FRA: filter retardation assay; fs: frameshift; HSPA/HSP70: heat shock protein family A (Hsp70); HSPB1/Hsp27: heat shock protein family B (small) member 1; HSPB8/Hsp22: heat shock protein family B (small) member 8; HTT: huntingtin; KO: knockout; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; MD: molecular dynamics; MTOC: microtubule organizing center; MYH: myosin heavy chain; MYOG: myogenin; NBR1: NBR1 autophagy cargo receptor; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; NSC34: Neuroblastoma X Spinal Cord 34; OPTN: optineurin; polyQ: polyglutamine; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; TARDBP/TDP-43: TAR DNA binding protein; TAX1BP1: Tax1 binding protein 1; TUBA: tubulin alpha; WT: wild-type.

The chaperone-assisted selective autophagy complex dynamics and dysfunctions

Each protein must be synthesized with the correct amino acid sequence, folded into its native structure, and transported to a relevant subcellular location and protein complex. If any of these steps fail, the cell has the capacity to break down aberrant proteins to maintain protein homeostasis (also called proteostasis). All cells possess a set of well-characterized protein quality control systems to minimize protein misfolding and the damage it might cause. Autophagy, a conserved pathway for the degradation of long-lived proteins, aggregates, and damaged organelles, was initially characterized as a bulk degradation pathway. However, it is now clear that autophagy also contributes to intracellular homeostasis by selectively degrading cargo material. One of the pathways involved in the selective removal of damaged and misfolded proteins is chaperone-assisted selective autophagy (CASA). The CASA complex is composed of three main proteins (HSPA, HSPB8 and BAG3), essential to maintain protein homeostasis in muscle and neuronal cells. A failure in the CASA complex, caused by mutations in the respective coding genes, can lead to (cardio)myopathies and neurodegenerative diseases. Here, we summarize our current understanding of the CASA complex and its dynamics. We also briefly discuss how CASA complex proteins are involved in disease and may represent an interesting therapeutic target.Abbreviation ALP: autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; AMOTL1: angiomotin like 1; ARP2/3: actin related protein 2/3; BAG: BAG cochaperone; BAG3: BAG cochaperone 3; CASA: chaperone-assisted selective autophagy; CMA: chaperone-mediated autophagy; DNAJ/HSP40: DnaJ heat shock protein family (Hsp40); DRiPs: defective ribosomal products; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK1/HRI: eukaryotic translation initiation factor 2 alpha kinase 1; GABARAP: GABA type A receptor-associated protein; HDAC6: histone deacetylase 6; HSP: heat shock protein; HSPA/HSP70: heat shock protein family A (Hsp70); HSP90: heat shock protein 90; HSPB8: heat shock protein family B (small) member 8; IPV: isoleucine-proline-valine; ISR: integrated stress response; KEAP1: kelch like ECH associated protein 1; LAMP2A: lysosomal associated membrane protein 2A; LATS1: large tumor suppressor kinase 1; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOC: microtubule organizing center; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-κB: nuclear factor kappa B; NFE2L2: NFE2 like bZIP transcription factor 2; PLCG/PLCγ: phospholipase C gamma; polyQ: polyglutamine; PQC: protein quality control; PxxP: proline-rich; RAN translation: repeat-associated non-AUG translation; SG: stress granule; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; STK: serine/threonine kinase; SYNPO: synaptopodin; TBP: TATA-box binding protein; TARDBP/TDP-43: TAR DNA binding protein; TFEB: transcription factor EB; TPR: tetratricopeptide repeats; TSC1: TSC complex subunit 1; UBA: ubiquitin associated; UPS: ubiquitin-proteasome system; WW: tryptophan-tryptophan; WWTR1: WW domain containing transcription regulator 1; YAP1: Yes1 associated transcriptional regulator.

A little less aggregation a little more replication: Viral manipulation of stress granules

Recent exciting studies have uncovered how membrane-less organelles, also known as biocondensates, are providing cells with rapid response pathways, allowing them to re-organize their cellular contents and adapt to stressful conditions. Their assembly is driven by the phase separation of their RNAs and intrinsically disordered protein components into condensed foci. Among these, stress granules (SGs) are dynamic cytoplasmic biocondensates that form in response to many stresses, including activation of the integrated stress response or viral infections. SGs sit at the crossroads between antiviral signaling and translation because they concentrate signaling proteins and components of the innate immune response, in addition to translation machinery and stalled mRNAs. Consequently, they have been proposed to contribute to antiviral activities, and therefore are targeted by viral countermeasures. Equally, SGs components can be commandeered by viruses for their own efficient replication. Phase separation processes are an important component of the viral life cycle, for example, driving the assembly of replication factories or inclusion bodies. Therefore, in this review, we will outline the recent understanding of this complex interplay and tug of war between viruses, SGs, and their components. This article is categorized under: RNA in Disease and Development > RNA in Disease Translation > Regulation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Identification of HSPB8 modulators counteracting misfolded protein accumulation in neurodegenerative diseases

AIMS

The small Heat Shock Protein B8 (HSPB8) is the core component of the chaperone-assisted selective autophagy (CASA) complex. This complex selectively targets, transports, and tags misfolded proteins for their recognition by autophagic receptors and insertion into autophagosome for clearance. CASA is essential to maintain intracellular proteostasis, especially in heart, muscle, and brain often exposed to various types of cell stresses. In neurons, HSPB8 protects against neurotoxicity caused by misfolded proteins in several models of neurodegenerative diseases; by facilitating autophagy, HSPB8 assists misfolded protein degradation also counteracting proteasome overwhelming and inhibition.

MATERIALS AND METHODS

To enhance HSPB8 protective activity, we screened a library of approximately 120,000 small molecules to identify compounds capable of increasing HSPB8 gene transcription, translation, or protein stability. We found 83 compounds active in preliminary dose-response assays and further classified them in 19 chemical classes by medicinal chemists' visual inspection. Of these 19 prototypes, 14 induced HSPB8 mRNA and protein levels in SH-SY5Y cells.

KEY FINDINGS

Out of these 14, 3 successfully reduced the aggregation propensity of a disease-associated mutant misfolded Superoxide Dismutase 1 (SOD1) protein in a flow cytometry-based "aggregation assay" [Flow cytometric analysis of Inclusions and Trafficking" (FloIT)] and induced the expression (mRNA and protein) of some autophagy receptors. Notably, the 3 hits were inactive in HSPB8-depleted cells, confirming that their protective activity is mediated by and requires HSPB8.

SIGNIFICANCE

Thus, these compounds may be highly relevant for a therapeutic approach in several human disorders, including neurodegenerative diseases, in which enhancement of CASA exerts beneficial activities.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.

The Role of Small Heat Shock Proteins in Protein Misfolding Associated Motoneuron Diseases

Motoneuron diseases (MNDs) are neurodegenerative conditions associated with death of upper and/or lower motoneurons (MNs). Proteostasis alteration is a pathogenic mechanism involved in many MNDs and is due to the excessive presence of misfolded and aggregated proteins. Protein misfolding may be the product of gene mutations, or due to defects in the translation process, or to stress agents; all these conditions may alter the native conformation of proteins making them prone to aggregate. Alternatively, mutations in members of the protein quality control (PQC) system may determine a loss of function of the proteostasis network. This causes an impairment in the capability to handle and remove aberrant or damaged proteins. The PQC system consists of the degradative pathways, which are the autophagy and the proteasome, and a network of chaperones and co-chaperones. Among these components, Heat Shock Protein 70 represents the main factor in substrate triage to folding, refolding, or degradation, and it is assisted in this task by a subclass of the chaperone network, the small heat shock protein (sHSPs/HSPBs) family. HSPBs take part in proteostasis by bridging misfolded and aggregated proteins to the HSP70 machinery and to the degradative pathways, facilitating refolding or clearance of the potentially toxic proteins. Because of its activity against proteostasis alteration, the chaperone system plays a relevant role in the protection against proteotoxicity in MNDs. Here, we discuss the role of HSPBs in MNDs and which HSPBs may represent a valid target for therapeutic purposes.

The Role and Therapeutic Potential of the Integrated Stress Response in Amyotrophic Lateral Sclerosis

In amyotrophic lateral sclerosis (ALS) patients, loss of cellular homeostasis within cortical and spinal cord motor neurons triggers the activation of the integrated stress response (ISR), an intracellular signaling pathway that remodels translation and promotes a gene expression program aimed at coping with stress. Beyond its neuroprotective role, under regimes of chronic or excessive stress, ISR can also promote cell/neuronal death. Given the two-edged sword nature of ISR, many experimental attempts have tried to establish the therapeutic potential of ISR enhancement or inhibition in ALS. This review discusses the complex interplay between ISR and disease progression in different models of ALS, as well as the opportunities and limitations of ISR modulation in the hard quest to find an effective therapy for ALS.

The role of eIF2 phosphorylation in cell and organismal physiology: new roles for well-known actors

Control of protein synthesis (mRNA translation) plays key roles in shaping the proteome and in many physiological, including homeostatic, responses. One long-known translational control mechanism involves phosphorylation of initiation factor, eIF2, which is catalysed by any one of four protein kinases, which are generally activated in response to stresses. They form a key arm of the integrated stress response (ISR). Phosphorylated eIF2 inhibits eIF2B (the protein that promotes exchange of eIF2-bound GDP for GTP) and thus impairs general protein synthesis. However, this mechanism actually promotes translation of certain mRNAs by virtue of specific features they possess. Recent work has uncovered many previously unknown features of this regulatory system. Several studies have yielded crucial insights into the structure and control of eIF2, including that eIF2B is regulated by several metabolites. Recent studies also reveal that control of eIF2 and the ISR helps determine organismal lifespan and surprising roles in sensing mitochondrial stresses and in controlling the mammalian target of rapamycin (mTOR). The latter effect involves an unexpected role for one of the eIF2 kinases, HRI. Phosphoproteomic analysis identified new substrates for another eIF2 kinase, Gcn2, which senses the availability of amino acids. Several genetic disorders arise from mutations in genes for eIF2α kinases or eIF2B (i.e. vanishing white matter disease, VWM and microcephaly, epileptic seizures, microcephaly, hypogenitalism, diabetes and obesity, MEHMO). Furthermore, the eIF2-mediated ISR plays roles in cognitive decline associated with Alzheimer's disease. New findings suggest potential therapeutic value in interfering with the ISR in certain settings, including VWM, for example by using compounds that promote eIF2B activity.

Regulation and function of elF2B in neurological and metabolic disorders

Eukaryotic initiation factor 2B, eIF2B is a guanine nucleotide exchange, factor with a central role in coordinating the initiation of translation. During stress and disease, the activity of eIF2B is inhibited via the phosphorylation of its substrate eIF2 (p-eIF2α). A number of different kinases respond to various stresses leading to the phosphorylation of the alpha subunit of eIF2, and collectively this regulation is known as the integrated stress response, ISR. This targeting of eIF2B allows the cell to regulate protein synthesis and reprogramme gene expression to restore homeostasis. Advances within structural biology have furthered our understanding of how eIF2B interacts with eIF2 in both the productive GEF active form and the non-productive eIF2α phosphorylated form. Here, current knowledge of the role of eIF2B in the ISR is discussed within the context of normal and disease states focusing particularly on diseases such as vanishing white matter disease (VWMD) and permanent neonatal diabetes mellitus (PNDM), which are directly linked to mutations in eIF2B. The role of eIF2B in synaptic plasticity and memory formation is also discussed. In addition, the cellular localisation of eIF2B is reviewed and considered along with the role of additional in vivo eIF2B binding factors and protein modifications that may play a role in modulating eIF2B activity during health and disease.

Heme-regulated inhibitor: an overlooked eIF2α kinase in cancer investigations

Heme-regulated inhibitor (HRI) kinase is a serine-threonine kinase, controlling the initiation of protein synthesis via phosphorylating α subunit of eIF2 on serine 51 residue, mainly in response to heme deprivation in erythroid cells. However, recent studies showed that HRI is also activated by several diverse signals, causing dysregulations in intracellular homeostatic mechanisms in non-erythroid cells. For instance, it was reported that the decrease in protein synthesis upon the 26S proteasomal inhibition by MG132 or bortezomib is mediated by increased eIF2α phosphorylation in an HRI-dependent manner in mouse embryonic fibroblast cells. The increase in eIF2α phosphorylation level through the activation of HRI upon 26S proteasomal inhibition is believed to protect cells against the buildup of misfolded and ubiquitinated proteins, having the potential to trigger the apoptotic response. In contrast, prolonged and sustained HRI-mediated eIF2α phosphorylation can induce cell death, which may involve ATF4 and CHOP expression. Altogether, these studies suggest that HRI-mediated eIF2α phosphorylation may be cytoprotective or cytotoxic depending on the cells, type, and duration of pharmacological agents used. It is thus hypothesized that both HRI activators, inducing eIF2α phosphorylation or HRI inhibitors causing disturbances in eIF2α phosphorylation, may be effective as novel strategies in cancer treatment if the balance in eIF2α phosphorylation is shifted in favor of autophagic or apoptotic response in cancer cells. It is here aimed to review the role of HRI in various biological mechanisms as well as the therapeutic potentials of recently developed HRI activators and inhibitors, targeting eIF2α phosphorylation in cancer cells.

Cytoplasmic proteotoxicity regulates HRI-dependent phosphorylation of eIF2α via the Hsp70-Bag3 module

The major heat shock protein Hsp70 forms a complex with a scaffold protein Bag3 that links it to components of signaling pathways. Via these interactions, the Hsp70-Bag3 module functions as a proteotoxicity sensor that controls cell signaling. Here, to search for pathways regulated by the complex, we utilized JG-98, an allosteric inhibitor of Hsp70 that blocks its interaction with Bag3. RNAseq followed by the pathway analysis indicated that several signaling pathways including UPR were activated by JG-98. Surprisingly, only the eIF2α-associated branch of the UPR was activated, while other UPR branches were not induced, suggesting that the response was unrelated to the ER proteotoxicity and ER-associated kinase PERK1. Indeed, induction of the UPR genes under these conditions was driven by a distinct eIF2α kinase HRI. Hsp70-Bag3 directly interacted with HRI and regulated eIF2α phosphorylation upon cytoplasmic proteotoxicity. Therefore, cytosolic proteotoxicity can activate certain UPR genes via Hsp70-Bag3-HRI-eIF2α axis.

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Disruption of Hsp70-Bag3 module activates the unfolded protein response (UPR)

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This induction of UPR genes is mediated by HRI-dependent phosphorylation of eIF2α

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Hsp70-Bag3 “monitors” cytoplasmic proteotoxicity to activate the HRI-eIF2α axis

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eIF2α integrates proteotoxicity signals from ER and cytoplasm

Insights on Human Small Heat Shock Proteins and Their Alterations in Diseases

The family of the human small Heat Shock Proteins (HSPBs) consists of ten members of chaperones (HSPB1-HSPB10), characterized by a low molecular weight and capable of dimerization and oligomerization forming large homo- or hetero-complexes. All HSPBs possess a highly conserved centrally located α-crystallin domain and poorly conserved N- and C-terminal domains. The main feature of HSPBs is to exert cytoprotective functions by preserving proteostasis, assuring the structural maintenance of the cytoskeleton and acting in response to cellular stresses and apoptosis. HSPBs take part in cell homeostasis by acting as holdases, which is the ability to interact with a substrate preventing its aggregation. In addition, HSPBs cooperate in substrates refolding driven by other chaperones or, alternatively, promote substrate routing to degradation. Notably, while some HSPBs are ubiquitously expressed, others show peculiar tissue-specific expression. Cardiac muscle, skeletal muscle and neurons show high expression levels for a wide variety of HSPBs. Indeed, most of the mutations identified in HSPBs are associated to cardiomyopathies, myopathies, and motor neuropathies. Instead, mutations in HSPB4 and HSPB5, which are also expressed in lens, have been associated with cataract. Mutations of HSPBs family members encompass base substitutions, insertions, and deletions, resulting in single amino acid substitutions or in the generation of truncated or elongated proteins. This review will provide an updated overview of disease-related mutations in HSPBs focusing on the structural and biochemical effects of mutations and their functional consequences.

Targeting the Integrated Stress Response in Cancer Therapy

The integrated stress response (ISR) is an evolutionarily conserved intra-cellular signaling network which is activated in response to intrinsic and extrinsic stresses. Various stresses are sensed by four specialized kinases, PKR-like ER kinase (PERK), general control non-derepressible 2 (GCN2), double-stranded RNA-dependent protein kinase (PKR) and heme-regulated eIF2α kinase (HRI) that converge on phosphorylation of serine 51 of eIF2α. eIF2α phosphorylation causes a global reduction of protein synthesis and triggers the translation of specific mRNAs, including activating transcription factor 4 (ATF4). Although the ISR promotes cell survival and homeostasis, when stress is severe or prolonged the ISR signaling will shift to regulate cellular apoptosis. We review the ISR signaling pathway, regulation and importance in cancer therapy.

Sensing, signaling and surviving mitochondrial stress

Mitochondrial fidelity is a key determinant of longevity and was found to be perturbed in a multitude of disease contexts ranging from neurodegeneration to heart failure. Tight homeostatic control of the mitochondrial proteome is a crucial aspect of mitochondrial function, which is severely complicated by the evolutionary origin and resulting peculiarities of the organelle. This is, on one hand, reflected by a range of basal quality control factors such as mitochondria-resident chaperones and proteases, that assist in import and folding of precursors as well as removal of aggregated proteins. On the other hand, stress causes the activation of several additional mechanisms that counteract any damage that may threaten mitochondrial function. Countermeasures depend on the location and intensity of the stress and on a range of factors that are equipped to sense and signal the nature of the encountered perturbation. Defective mitochondrial import activates mechanisms that combat the accumulation of precursors in the cytosol and the import pore. To resolve proteotoxic stress in the organelle interior, mitochondria depend on nuclear transcriptional programs, such as the mitochondrial unfolded protein response and the integrated stress response. If organelle damage is too severe, mitochondria signal for their own destruction in a process termed mitophagy, thereby preventing further harm to the mitochondrial network and allowing the cell to salvage their biological building blocks. Here, we provide an overview of how different types and intensities of stress activate distinct pathways aimed at preserving mitochondrial fidelity.

Unique integrated stress response sensors regulate cancer cell susceptibility when Hsp70 activity is compromised

Molecular chaperones, such as Hsp70, prevent proteotoxicity and maintain homeostasis. This is perhaps most evident in cancer cells, which overexpress Hsp70 and thrive even when harboring high levels of misfolded proteins. To define the response to proteotoxic challenges, we examined adaptive responses in breast cancer cells in the presence of an Hsp70 inhibitor. We discovered that the cells bin into distinct classes based on inhibitor sensitivity. Strikingly, the most resistant cells have higher autophagy levels, and autophagy was maximally activated only in resistant cells upon Hsp70 inhibition. In turn, resistance to compromised Hsp70 function required the integrated stress response transducer, GCN2, which is commonly associated with amino acid starvation. In contrast, sensitive cells succumbed to Hsp70 inhibition by activating PERK. These data reveal an unexpected route through which breast cancer cells adapt to proteotoxic insults and position GCN2 and autophagy as complementary mechanisms to ensure survival when proteostasis is compromised.