Opposite Roles of RNase and Kinase Activities of Inositol-Requiring Enzyme 1 (IRE1) on HSV-1 Replication

Neurotropic virus infection and neurodegenerative diseases: Potential roles of autophagy pathway

Neurodegenerative diseases (NDs) constitute a group of disorders characterized by the progressive deterioration of nervous system functionality. Currently, the precise etiological factors responsible for NDs remain incompletely elucidated, although it is probable that a combination of aging, genetic predisposition, and environmental stressors participate in this process. Accumulating evidence indicates that viral infections, especially neurotropic viruses, can contribute to the onset and progression of NDs. In this review, emerging evidence supporting the association between viral infection and NDs is summarized, and how the autophagy pathway mediated by viral infection can cause pathological aggregation of cellular proteins associated with various NDs is discussed. Furthermore, autophagy-related genes (ARGs) involved in Herpes simplex virus (HSV-1) infection and NDs are analyzed, and whether these genes could link HSV-1 infection to NDs is discussed. Elucidating the mechanisms underlying NDs is critical for developing targeted therapeutic approaches that prevent the onset and slow the progression of NDs.

Manipulation of unfolded protein response by zoonotic vaccinia virus strains Guarani P1 and Passatempo

The replicative success of vaccinia virus (VACV) depends on its ability to subvert host functions. Poxviruses multiplication and maturation are closely associated with the endoplasmic reticulum (ER) and its membranes. This organelle responds to disturbances caused by the accumulation of misfolded proteins, leading to processing of these proteins or even programmed cell death through the unfolded protein response (UPR). Several studies show that different viruses can activate UPR pathway components and negatively modulate others. Here, we investigate the effects of infections by zoonotic VACV strains from Brazil, Guarani P1 virus (GP1V) and Passatempo virus (PSTV), in the activation of UPR pathway sensors. We observed translocation of ATF6 to the nucleus as well as transcriptional increase after GP1V, PSTV, and reference strain Western Reserve (WR) infection. XBP1 processing appears to be negatively modulated after VACV infection; however, inhibition of the inositol-requiring enzyme 1 (IRE1) kinase domain led to a reduction in plaque sizes for these viruses. The absence of PKR-like endoplasmic reticulum kinase (PERK) has an impact on the plaque phenotype of GP1V, PSTV viruses, as well as for the prototypical strain WR. These results indicate that the VACV manipulates the three arms of the UPR path differently to ensure replicative success.

The Unfolded Protein Response Sensor IRE1 Regulates Activation of In Vitro Differentiated Type 1 Conventional DCs with Viral Stimuli

Type 1 conventional dendritic cells (cDC1s) are leukocytes competent to coordinate antiviral immunity, and thus, the intracellular mechanisms controlling cDC1 function are a matter of intense research. The unfolded protein response (UPR) sensor IRE1 and its associated transcription factor XBP1s control relevant functional aspects in cDC1s including antigen cross-presentation and survival. However, most studies connecting IRE1 and cDC1 function are undertaken in vivo. Thus, the aim of this work is to elucidate whether IRE1 RNase activity can also be modeled in cDC1s differentiated in vitro and reveal the functional consequences of such activation in cells stimulated with viral components. Our data show that cultures of optimally differentiated cDC1s recapitulate several features of IRE1 activation noticed in in vivo counterparts and identify the viral analog Poly(I:C) as a potent UPR inducer in the lineage. In vitro differentiated cDC1s display constitutive IRE1 RNase activity and hyperactivate IRE1 RNase upon genetic deletion of XBP1s, which regulates production of the proinflammatory cytokines IL-12p40, TNF-α and IL-6, Ifna and Ifnb upon Poly(I:C) stimulation. Our results show that a strict regulation of the IRE1/XBP1s axis regulates cDC1 activation to viral agonists, expanding the scope of this UPR branch in potential DC-based therapies.

Live and let die: signaling AKTivation and UPRegulation dynamics in SARS-CoVs infection and cancer

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the pathogen responsible for the coronavirus disease 2019 (COVID-19) pandemic. Of particular interest for this topic are the signaling cascades that regulate cell survival and death, two opposite cell programs whose control is hijacked by viral infections. The AKT and the Unfolded Protein Response (UPR) pathways, which maintain cell homeostasis by regulating these two programs, have been shown to be deregulated during SARS-CoVs infection as well as in the development of cancer, one of the most important comorbidities in relation to COVID-19. Recent evidence revealed two way crosstalk mechanisms between the AKT and the UPR pathways, suggesting that they might constitute a unified homeostatic control system. Here, we review the role of the AKT and UPR pathways and their interaction in relation to SARS-CoV-2 infection as well as in tumor onset and progression. Feedback regulation between AKT and UPR pathways emerges as a master control mechanism of cell decision making in terms of survival or death and therefore represents a key potential target for developing treatments for both viral infection and cancer. In particular, drug repositioning, the investigation of existing drugs for new therapeutic purposes, could significantly reduce time and costs compared to de novo drug discovery.

FKBP11 rewires UPR signaling to promote glucose homeostasis in type 2 diabetes and obesity

Chronic endoplasmic reticulum (ER) stress and sustained activation of unfolded protein response (UPR) signaling contribute to the development of type 2 diabetes in obesity. UPR signaling is a complex signaling pathway, which is still being explored in many different cellular processes. Here, we demonstrate that FK506-binding protein 11 (FKBP11), which is transcriptionally regulated by XBP1s, is severely reduced in the livers of obese mice. Restoring hepatic FKBP11 expression in obese mice initiates an atypical UPR signaling pathway marked by rewiring of PERK signaling toward NRF2, away from the eIF2α-ATF4 axis of the UPR. This alteration in UPR signaling establishes glucose homeostasis without changing hepatic ER stress, food consumption, or body weight. We conclude that ER stress during obesity can be beneficially rewired to promote glucose homeostasis. These findings may uncover possible new avenues in the development of novel approaches to treat diseases marked by ER stress.

Pharmacological Inhibition of IRE-1 Alpha Activity in Herpes Simplex Virus Type 1 and Type 2-Infected Dendritic Cells Enhances T Cell Activation

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) infections are life-long and highly prevalent in the human population. These viruses persist in the host, eliciting either symptomatic or asymptomatic infections that may occur sporadically or in a recurrent manner through viral reactivations. Clinical manifestations due to symptomatic infection may be mild such as orofacial lesions, but may also translate into more severe diseases, such as ocular infections that may lead to blindness and life-threatening encephalitis. A key feature of herpes simplex viruses (HSVs) is that they have evolved molecular determinants that hamper numerous components of the host’s antiviral innate and adaptive immune system. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), by inhibiting their T cell-activating capacity and eliciting their apoptosis after infection. Previously, we reported that HSV-2 activates the splicing of the mRNA of XBP1, which is related to the activity of the unfolded protein response (UPR) factor Inositol-Requiring Enzyme 1 alpha (IRE-1α). Here, we sought to evaluate if the activation of the IRE-1α pathway in DCs upon HSV infection may be related to impaired DC function after infection with HSV-1 or HSV-2. Interestingly, the pharmacological inhibition of the endonuclease activity of IRE-1α in HSV-1- and HSV-2-infected DCs significantly reduced apoptosis in these cells and enhanced their capacity to migrate to lymph nodes and activate virus-specific CD4⁺ and CD8⁺ T cells. These findings suggest that the activation of the IRE-1α-dependent UPR pathway in HSV-infected DCs may play a significant role in the negative effects that these viruses exert over these cells and that the modulation of this signaling pathway may be relevant for enhancing the function of DCs upon infection with HSVs.

Herpes simplex virus interference with immunity: Focus on dendritic cells

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent in the human population. These viruses cause lifelong infections by establishing latency in neurons and undergo sporadic reactivations that promote recurrent disease and new infections. The success of HSVs in persisting in infected individuals is likely due to their multiple molecular determinants involved in escaping the host antiviral and immune responses. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), key immune cells that are involved in establishing effective and balanced immunity against viruses. Here, we review and discuss several molecular and cellular processes modulated by HSVs in DCs, such as autophagy, apoptosis, and the unfolded protein response. Given the central role of DCs in establishing optimal antiviral immunity, particular emphasis should be given to the outcome of the interactions occurring between HSVs and DCs.

Anti-viral activity of Staphylococcus aureus lysates against herpes simplex virus type-I infection: an in vitro and in vivo study

AIM

To investigate the effect of Staphylococcus aureus (S. aures) lysates (SALs) on herpes simplex virus type-I (HSV1) infection in human corneal epithelial (HCE) cells and in a mouse model of HSV1 keratitis.

METHODS

HCE, Vero, HeLa, and BV2 cells were infected with HSV1 [HSV1 f strain, HSV1f; HSV-1-H129 with green fluorescent protein (GFP) knock-in, HSV1g]. Pre- or post-infection, SAL at various concentrations was added to the culture medium for 24h. GFP fluorescence in HSV1g or plaque formation by HSV1f were examined. The effects of heat-treated SAL, precooled acetone-precipitated SAL, and SAL subjected to ultrafiltration (100 kDa) were evaluated. The effects of other bacterial components and lysates on HSV1 infection were also tested, including lipoteichoic acid (LTA), peptidoglycan (PGN), staphylococcal protein A (SPA), and α-hemolysin from S. aureus (α-toxin) as well as lysates from a wild-type S. aureus strain, S. epidermidis, and Escherichia coli (W-SAL, SEL, and ECL, respectively). In addition, SAL eye drops were applied topically to BALB/c mice with HSV1 keratitis, followed by in vivo observations.

RESULTS

The cytopathic effect, plaque formation (HSV1f), and GFP expression (HSV1g) in infected cells were inhibited by SAL in a dose-dependent manner. The active component of SAL (≥100 kDa) was heat-sensitive and retained activity after acetone precipitation. In HSV1g-infected cells, treatment with LTA-sa, α-toxin, PGN-sa, or SPA did not inhibit GFP expression. SAL, W-SAL, and SEL (but not ECL) decreased GFP expression. In mice with HSV1 keratitis, SAL reduced corneal lesions by 71%.

CONCLUSION

The results of this study demonstrate that SAL can be used to inhibit HSV1 infection, particularly keratitis. Further studies are needed to determine the active components and mechanism underlying the effects of SAL.

Inflammation promotes adipocyte lipolysis via IRE1 kinase

Obesity associates with inflammation, insulin resistance, and higher blood lipids. It is unclear if immune responses facilitate lipid breakdown and release from adipocytes via lipolysis in a separate way from hormones or adrenergic signals. We found that an ancient component of ER stress, inositol-requiring protein 1 (IRE1), discriminates inflammation-induced adipocyte lipolysis versus lipolysis from adrenergic or hormonal stimuli. Our data show that inhibiting IRE1 kinase activity was sufficient to block adipocyte-autonomous lipolysis from multiple inflammatory ligands, including bacterial components, certain cytokines, and thapsigargin-induced ER stress. IRE1-mediated lipolysis was specific for inflammatory triggers since IRE1 kinase activity was dispensable for isoproterenol and cAMP-induced lipolysis in adipocytes and mouse adipose tissue. IRE1 RNase activity was not associated with inflammation-induced adipocyte lipolysis. Inhibiting IRE1 kinase activity blocked NF-κB activation, interleukin-6 secretion, and adipocyte-autonomous lipolysis from inflammatory ligands. Inflammation-induced lipolysis mediated by IRE1 occurred independently from changes in insulin signaling in adipocytes, suggesting that inflammation can promote IRE1-mediated lipolysis independent of adipocyte insulin resistance. We found no role for canonical unfolded protein responses or ABL kinases in linking ER stress to IRE1-mediated lipolysis. Adiponectin-Cre-mediated IRE1 knockout in mice showed that adipocyte IRE1 was required for inflammatory ligand-induced lipolysis in adipose tissue explants and that adipocyte IRE1 was required for approximately half of the increase in blood triglycerides after a bacterial endotoxin-mediated inflammatory stimulus in vivo. Together, our results show that IRE1 propagates an inflammation-specific lipolytic program independent from hormonal or adrenergic regulation. Targeting IRE1 kinase activity may benefit metabolic syndrome and inflammatory lipid disorders.

Activation of the IRE1 RNase through remodeling of the kinase front pocket by ATP-competitive ligands

Inositol-Requiring Enzyme 1 (IRE1) is an essential component of the Unfolded Protein Response. IRE1 spans the endoplasmic reticulum membrane, comprising a sensory lumenal domain, and tandem kinase and endoribonuclease (RNase) cytoplasmic domains. Excess unfolded proteins in the ER lumen induce dimerization and oligomerization of IRE1, triggering kinase trans-autophosphorylation and RNase activation. Known ATP-competitive small-molecule IRE1 kinase inhibitors either allosterically disrupt or stabilize the active dimeric unit, accordingly inhibiting or stimulating RNase activity. Previous allosteric RNase activators display poor selectivity and/or weak cellular activity. In this study, we describe a class of ATP-competitive RNase activators possessing high selectivity and strong cellular activity. This class of activators binds IRE1 in the kinase front pocket, leading to a distinct conformation of the activation loop. Our findings reveal exquisitely precise interdomain regulation within IRE1, advancing the mechanistic understanding of this important enzyme and its investigation as a potential small-molecule therapeutic target.

The RNase activity of Inositol-Requiring Enzyme 1 (IRE1) can be allosterically regulated by ATP-competitive inhibitors of the IRE1 kinase domain. Here, the authors identify ATP-competitive IRE1 RNase activators with improved selectivity and cellular activity, and elucidate their mechanism of action.

TUDCA inhibits HSV-1 replication by the modulating unfolded protein response pathway

Tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, was used to protect liver function through antiapoptosis or reducing endoplasmic reticulum stress (ER stress). Previous studies showed that ER stress was modulated by herpes simplex virus types 1 (HSV-1) infection to facilitate viral replication. Here, we investigated the effect of TUDCA on HSV-1 infection of HEC-1-A cells and showed that both replication and multiplication of the virus were inhibited by TUDCA in a dose dependent manner. Unfolded protein response was induced to deliver stress signals from ER to nucleus. We found that TUDCA alleviated activating transcription factor 6 branch inhibition, partially enhanced protein kinase RNA-like ER kinase pathway activation, and repressed inositol-requiring protein 1α arm activation significantly in infected cells. The findings of this study suggest that TUDCA inhibits HSV-1 replication through ER stress pathway, which may provide a potential therapeutic strategy for HSV-1 infection.

Repurposing Sigma-1 Receptor Ligands for COVID-19 Therapy?

Outbreaks of emerging infections, such as COVID-19 pandemic especially, confront health professionals with the unique challenge of treating patients. With no time to discover new drugs, repurposing of approved drugs or in clinical development is likely the only solution. Replication of coronaviruses (CoVs) occurs in a modified membranous compartment derived from the endoplasmic reticulum (ER), causes host cell ER stress and activates pathways to facilitate adaptation of the host cell machinery to viral needs. Accordingly, modulation of ER remodeling and ER stress response might be pivotal in elucidating CoV-host interactions and provide a rationale for new therapeutic, host-based antiviral approaches. The sigma-1 receptor (Sig-1R) is a ligand-operated, ER membrane-bound chaperone that acts as an upstream modulator of ER stress and thus a candidate host protein for host-based repurposing approaches to treat COVID-19 patients. Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against CoVs, including severe acute respiratory syndrome CoV-2 (SARS-CoV-2). Sig-1R regulates key mechanisms of the adaptive host cell stress response and takes part in early steps of viral replication. It is enriched in lipid rafts and detergent-resistant ER membranes, where it colocalizes with viral replicase proteins. Indeed, the non-structural SARS-CoV-2 protein Nsp6 interacts with Sig-1R. The activity of Sig-1R ligands against COVID-19 remains to be specifically assessed in clinical trials. This review provides a rationale for targeting Sig-1R as a host-based drug repurposing approach to treat COVID-19 patients. Evidence gained using Sig-1R ligands in unbiased in vitro antiviral drug screens and the potential mechanisms underlying the modulatory effect of Sig-1R on the host cell response are discussed. Targeting Sig-1R is not expected to reduce dramatically established viral replication, but it might interfere with early steps of virus-induced host cell reprogramming, aid to slow down the course of infection, prevent the aggravation of the disease and/or allow a time window to mature a protective immune response. Sig-1R-based medicines could provide benefit not only as early intervention, preventive but also as adjuvant therapy.

Induction of the Unfolded Protein Response during Bovine Alphaherpesvirus 1 Infection

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes great economic losses in the cattle industry. Herpesvirus infection generally induces endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in infected cells. However, it is not clear whether ER stress and UPR can be induced by BoHV-1 infection. Here, we found that ER stress induced by BoHV-1 infection could activate all three UPR sensors (the activating transcription factor 6 (ATF6), the inositol-requiring enzyme 1 (IRE1), and the protein kinase RNA-like ER kinase (PERK)) in MDBK cells. During BoHV-1 infection, the ATF6 pathway of UPR did not affect viral replication. However, both knockdown and specific chemical inhibition of PERK attenuated the BoHV-1 proliferation, and chemical inhibition of PERK significantly reduced the viral replication at the post-entry step of the BoHV-1 life cycle. Furthermore, knockdown of IRE1 inhibits BoHV-1 replication, indicating that the IRE1 pathway may promote viral replication. Further study revealed that BoHV-1 replication was enhanced by IRE1 RNase activity inhibition at the stage of virus post-entry in MDBK cells. Furthermore, IRE1 kinase activity inhibition and RNase activity enhancement decrease BoHV1 replication via affecting the virus post-entry step. Our study revealed that BoHV-1 infection activated all three UPR signaling pathways in MDBK cells, and BoHV-1-induced PERK and IRE1 pathways may promote viral replication. This study provides a new perspective for the interactions of BoHV-1 and UPR, which is helpful to further elucidate the mechanism of BoHV-1 pathogenesis.

Multiple Herpes Simplex Virus-1 (HSV-1) Reactivations Induce Protein Oxidative Damage in Mouse Brain: Novel Mechanisms for Alzheimer's Disease Progression

Compelling evidence supports the role of oxidative stress in Alzheimer's disease (AD) pathophysiology. Interestingly, Herpes simplex virus-1 (HSV-1), a neurotropic virus that establishes a lifelong latent infection in the trigeminal ganglion followed by periodic reactivations, has been reportedly linked both to AD and to oxidative stress conditions. Herein, we analyzed, through biochemical and redox proteomic approaches, the mouse model of recurrent HSV-1 infection we previously set up, to investigate whether multiple virus reactivations induced oxidative stress in the mouse brain and affected protein function and related intracellular pathways. Following multiple HSV-1 reactivations, we found in mouse brains increased levels of oxidative stress hallmarks, including 4-hydroxynonenal (HNE), and 13 HNE-modified proteins whose levels were found significantly altered in the cortex of HSV-1-infected mice compared to controls. We focused on two proteins previously linked to AD pathogenesis, i.e., glucose-regulated protein 78 (GRP78) and collapsin response-mediated protein 2 (CRMP2), which are involved in the unfolded protein response (UPR) and in microtubule stabilization, respectively. We found that recurrent HSV-1 infection disables GRP78 function and activates the UPR, whereas it prevents CRMP2 function in mouse brains. Overall, these data suggest that repeated HSV-1 reactivation into the brain may contribute to neurodegeneration also through oxidative damage.

Host shutoff activity of VHS and SOX-like proteins: role in viral survival and immune evasion

BACKGROUND

Host shutoff refers to the widespread downregulation of host gene expression and has emerged as a key process that facilitates the reallocation of cellular resources for viral replication and evasion of host antiviral immune responses.

MAIN BODY

The Herpesviridae family uses a number of proteins that are responsible for host shutoff by directly targeting messenger RNA (mRNA), including virion host shutoff (VHS) protein and the immediate-early regulatory protein ICP27 of herpes simplex virus types 1 (HSV-1) and the SOX (shutoff and exonuclease) protein and its homologs in Gammaherpesvirinae subfamilies, although these proteins are not homologous. In this review, we highlight evidence that host shutoff is promoted by the VHS, ICP27 and SOX-like proteins and that they also contribute to immune evasion.

CONCLUSIONS

Further studies regarding the host shutoff proteins will not only contribute to provide new insights into the viral replication, expression and host immune evasion process, but also provide new molecular targets for the development of antiviral drugs and therapies.

Marburg virus regulates the IRE1/XBP1-dependent unfolded protein response to ensure efficient viral replication

Viruses regulate cellular signalling pathways to ensure optimal viral replication. During Marburg virus (MARV) infection, large quantities of the viral glycoprotein GP are produced in the ER; this may result in the activation of the unfolded protein response (UPR). The most conserved pathway to trigger UPR is initiated by IRE1. Activation of IRE1 results in auto-phosphorylation, splicing of the XBP1 mRNA and translation of the XBP1s protein. XBP1s binds cis-acting UPR elements (UPRE) which leads to the enhanced expression of genes which should restore ER homeostasis. XBP1u protein is translated, if IRE1 is not activated. Here we show that ectopic expression of MARV GP activated the IRE1-XBP1 axis of UPR as monitored by UPRE luciferase assays. However, while at 24 h of infection with MARV IRE1 was phosphorylated, expression of XBP1s was only slightly enhanced and UPRE activity was not detected. The IRE1-XBP1 axis was not active at 48 h p.i. Co-expression studies of MARV proteins demonstrated that the MARV protein VP30 suppressed UPRE activation. Co-immunoprecipitation analyses revealed an RNA-dependent interaction of VP30 with XBP1u. Knock-out of IRE1 supported MARV infection at late time points. Taken together, these results suggest that efficient MARV propagation requires specific regulation of IRE1 activity.

Herpesviruses and the Unfolded Protein Response

Herpesviruses usurp cellular stress responses to promote viral replication and avoid immune surveillance. The unfolded protein response (UPR) is a conserved stress response that is activated when the protein load in the ER exceeds folding capacity and misfolded proteins accumulate. The UPR aims to restore protein homeostasis through translational and transcriptional reprogramming; if homeostasis cannot be restored, the UPR switches from "helper" to "executioner", triggering apoptosis. It is thought that the burst of herpesvirus glycoprotein synthesis during lytic replication causes ER stress, and that these viruses may have evolved mechanisms to manage UPR signaling to create an optimal niche for replication. The past decade has seen considerable progress in understanding how herpesviruses reprogram the UPR. Here we provide an overview of the molecular events of UPR activation, signaling and transcriptional outputs, and highlight key evidence that herpesviruses hijack the UPR to aid infection.

KSHV activates unfolded protein response sensors but suppresses downstream transcriptional responses to support lytic replication

Herpesviruses usurp host cell protein synthesis machinery to convert viral mRNAs into proteins, and the endoplasmic reticulum (ER) to ensure proper folding, post-translational modification and trafficking of secreted and transmembrane viral proteins. Overloading ER folding capacity activates the unfolded protein response (UPR), whereby sensor proteins ATF6, PERK and IRE1 initiate a stress-mitigating transcription program that accelerates catabolism of misfolded proteins while increasing ER folding capacity. Kaposi’s sarcoma-associated herpesvirus (KSHV) can be reactivated from latency by chemical induction of ER stress, which causes accumulation of the XBP1s transcription factor that transactivates the viral RTA lytic switch gene. The presence of XBP1s-responsive elements in the RTA promoter suggests that KSHV evolved a mechanism to respond to ER stress. Here, we report that ATF6, PERK and IRE1 were activated upon reactivation from latency and required for efficient KSHV lytic replication; genetic or pharmacologic inhibition of each UPR sensor diminished virion production. Despite UPR sensor activation during KSHV lytic replication, downstream UPR transcriptional responses were restricted; 1) ATF6 was cleaved to activate the ATF6(N) transcription factor but ATF6(N)-responsive genes were not transcribed; 2) PERK phosphorylated eIF2α but ATF4 did not accumulate; 3) IRE1 caused XBP1 mRNA splicing, but XBP1s protein did not accumulate and XBP1s-responsive genes were not transcribed. Ectopic expression of the KSHV host shutoff protein SOX did not affect UPR gene expression, suggesting that alternative viral mechanisms likely mediate UPR suppression during lytic replication. Complementation of XBP1s deficiency during KSHV lytic replication inhibited virion production in a dose-dependent manner in iSLK.219 cells but not in TREx-BCBL1-RTA cells. However, genetically distinct KSHV virions harvested from these two cell lines were equally susceptible to XBP1s restriction following infection of naïve iSLK cells. This suggests that cell-intrinsic properties of BCBL1 cells may circumvent the antiviral effect of ectopic XBP1s expression. Taken together, these findings indicate that while XBP1s plays an important role in reactivation from latency, it can inhibit virus replication at a later step, which the virus overcomes by preventing its synthesis. These findings suggest that KSHV hijacks UPR sensors to promote efficient viral replication while sustaining ER stress.

Like all viruses, Kaposi’s sarcoma-associated herpesvirus (KSHV) uses cellular machinery to create viral proteins. Some of these proteins are folded and modified in the endoplasmic reticulum (ER) and traverse the cellular secretory apparatus. Exceeding ER protein folding capacity activates the unfolded protein response (UPR), which resolves ER stress by putting the brakes on protein synthesis and turning on stress-mitigating genes. We show that KSHV replication activates the three cellular proteins that sense ER stress, which are each required to support efficient viral replication. By contrast, KSHV blocks the UPR gene expression program downstream from each of these activated sensor proteins. The failure to resolve ER stress might normally be expected to put the virus at a disadvantage, but we demonstrate that reversal of this scenario is worse; when we supplement infected epithelial cells with the UPR transcription factor XBP1s to artificially stimulate the production of UPR-responsive gene products, virus replication is blocked at a late stage and very few viruses are released from infected cells. Taken together, these observations suggest that KSHV requires UPR sensor protein activation to replicate but has dramatically altered the outcome to prevent the synthesis of new UPR proteins and sustain stress in the ER compartment.

T-2 toxin inhibits the production of mucin via activating the IRE1/XBP1 pathway

T-2 toxin is a trichothecene mycotoxin that widely contaminates food and has a variety of toxic effects. However, the underlying mechanism of T-2 toxin on intestinal mucin remains unclear. In present study, human intestinal Caco-2 cells and HT-29 cells were treated with 100 ng/mL T-2 toxin at one-quarter of the IC₅₀ for 24 h, which caused the inhibition of MUC2 and adhesion of E. coli O157:H7. We found T-2 toxin induced endoplasmic reticulum stress and activated the IRE1/XBP1 pathway, which may be related to the inhibition of MUC2. Interestingly, T-2 toxin activated IRE1α to inhibit IRE1β, which optimized mucin production. Furthermore, overexpression of IRE1β in the cells apparently alleviated the inhibition of MUC2 caused by T-2 toxin. IRE1α knock-down blocked the down-regulation of IRE1β and MUC2 induced by T-2 toxin. We revealed the critical role of IRE1α in the inhibition of intestinal mucin. This finding was confirmed in BALB/c mice which were exposed to T-2 toxin (0.5 mg/kg bw) for 4 weeks. T-2 toxin activated the IRE1/XBP1 pathway to disrupt intestinal mucin, which lead to the imbalance of gut microbiota and an increased risk of host infection by E. coli O157:H7. T-2 toxin exposure also increased the expressions of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α in mice, which might respond to IRE1α activation. Importantly, IRE1α activation was a therapeutic target for intestinal inflammation caused by T-2 toxin. This study provided a new perspective to understand the intestinal toxicity of T-2 toxin.

The unknown face of IRE1α - Beyond ER stress

IRE1α (Inositol Requiring kinase Enzyme 1 alpha), a transmembrane protein localized to the endoplasmic reticulum (ER) is a master regulator of the unfolded protein response (UPR) pathway. The fate determining steps during ER stress-induced apoptosis are greatly attributed to IRE1α's endoribonuclease and kinase activities. Apart from its role as a chief executioner in ER stress, recent studies have shown that upon activation in the presence or absence of ER stress, IRE1α executes multiple cellular processes such as differentiation, immune response, progression and repression of the cell cycle. Besides its crucial role in protein misfolding, the versatile contributions of IRE1α in other cellular functions are greatly unknown. In this review, we have discussed the structural conservation of IRE1 among eukaryotes, the mechanisms underlying its activation and the recent understandings of the non-apoptotic functions of IRE1 other than ER stress-induced cell death.