Mario Köster

Review: Activities of IRF-1

Interferon (IFN) regulatory factor-1 (IRF-1) was isolated by virtue of its affinity to specific DNA sequences in the IFN-β promoter that mediate virus responsiveness. IRF-1 was the first factor identified of the IRF family and was most extensively characterized at the molecular level. Also, its physiologic role in host defense against pathogens, tumor prevention, and development of the immune system was investigated in detail. Even though some of the functions first associated with IRF-1 were later found to be mediated in part or predominantly by other activators of the IRF family of transcription factors, IRF-1 has remained a central paradigm in the transcriptional regulation of the IFN response.

Multi-layered stochasticity and paracrine signal propagation shape the type-I interferon response

The cellular recognition of viruses evokes the secretion of type‐I interferons (IFNs) that induce an antiviral protective state. By live‐cell imaging, we show that key steps of virus‐induced signal transduction, IFN‐β expression, and induction of IFN‐stimulated genes (ISGs) are stochastic events in individual cells. The heterogeneity in IFN production is of cellular—and not viral—origin, and temporal unpredictability of IFN‐β expression is largely due to cell‐intrinsic noise generated both upstream and downstream of the activation of nuclear factor‐κB and IFN regulatory factor transcription factors. Subsequent ISG induction occurs as a stochastic all‐or‐nothing switch, where the responding cells are protected against virus replication. Mathematical modelling and experimental validation show that reliable antiviral protection in the face of multi‐layered cellular stochasticity is achieved by paracrine response amplification. Achieving coherent responses through intercellular communication is likely to be a more widely used strategy by mammalian cells to cope with pervasive stochasticity in signalling and gene expression.

Temporal and Spatial Resolution of Type I and III Interferon Responses In Vivo

ABSTRACT Although the action of interferons (IFNs) has been extensively studied in vitro, limited information is available on the spatial and temporal activation pattern of IFN-induced genes in vivo. We created BAC transgenic mice expressing firefly luciferase under transcriptional control of the Mx2 gene promoter. Expression of the reporter with regard to onset and kinetics of induction parallels that of Mx2 and is thus a hallmark for the host response. Substantial constitutive expression of the reporter gene was observed in the liver and most other tissues of transgenic mice, whereas this expression was strongly reduced in animals lacking functional type I IFN receptors. As expected, the reporter gene was induced not only in response to type I (α and β) and type III (λ) IFNs but also in response to a variety of IFN inducers such as double-stranded RNA, lipopolysaccharide (LPS), and viruses. In vivo IFN subtypes show clear differences with respect to their kinetics of action and to their spatial activation pattern: while the type I IFN response was strong in liver, spleen, and kidney, type III IFN reactivity was most prominent in organs with mucosal surfaces. Infection of reporter mice with virus strains that differ in their pathogenicity shows that the IFN response is significantly altered in the strength of IFN action at sites which are not primarily infected as well as by the onset and duration of gene induction.

Ratjadones inhibit nuclear export by blocking CRM1/exportin 1.

In addition to previously isolated ratjadone A we describe three new members of this family, ratjadones B, C, and D, from another strain of the myxobacterium Sorangium cellulosum. We have investigated the properties of these ratjadones with respect to their activity on mammalian cell lines. We found IC(50) values in the picomolar range and a significant increase in the size of nuclei. A further examination showed that they inhibit the export of the leucine-rich nuclear export signal (LR-NES) containing proteins in different cell lines. Ratjadones are able to inhibit the formation of the nuclear export complex composed of the CRM1, RanGTP, and the cargo protein, as shown by two different in vitro assays. Finally, the binding of ratjadone C to CRM1 was demonstrated. These ratjadone activities are in the same concentration range as described for the polyketide leptomycin B (LMB) from Streptomyces sp. Like LMB, it seems that the ratjadones covalently bind to CRM1, inhibit cargo protein binding via LR-NES, and thereby block nuclear export. Thus, the ratjadones represent a new class of natural compounds which inhibit proliferation in eukaryotes by blocking nuclear export.

Nucleolar localization and mobility analysis of the NF-κB repressing factor NRF

NF-κB plays a central role in mediating pathogen and cytokine-stimulated gene transcription. NF-κB repressing factor (NRF) has been shown to interact with specific negative regulatory DNA elements (NRE) to mediate transcriptional repression by inhibition of the NF-κB activity at certain promoters. mRNA ablation experiments demonstrated that the trans-acting NRF protein is involved in constitutive but not post-stimulated silencing of IFN-β, IL-8 and iNOS genes by binding to cis-acting NRE elements in their promoters. We have examined the subcellular localization and mobility of the NRF protein. Since neither tagging nor overexpression perturbs NRF localization the GFP-tagged protein was used for detailed localization and mobility studies. Owing to an N-terminal nuclear localization sequence, all NRF fragments that contain this signal show a constitutive nuclear accumulation. C-terminal NRF fragments also localize to the nucleus although no canonical NLS motifs were detected. Full-length NRF is highly enriched in nucleoli and only a small fraction of NRF is found in the nucleoplasm and cytoplasm. This relationship was found to be independent of the protein expression rate. FRAP analysis proved to be a sensitive method to determine protein mobility and made it possible to differentiate between the NRF protein fragments. Nucleolar localization correlated inversely with mobility. The data demonstrate that a series of neighboring fragments in a large central domain of the protein contribute to the strong nucleolar affinity. These properties were not altered by viral infection or LPS treatment. Several sequence motifs for RNA binding were predicted by computer-mediated databank searches. We found that NRF binds to double stranded RNA (dsRNA). This property mapped to several NRF fragments which correlate with the nucleolar affinity domain. Since treatment with actinomycin D releases NRF from nucleoli the identified RNA binding motifs might act as nucleolar localization signals.

Regulation of cell growth by IRF-1 in BHK-21 cells.

Most cell lines that are used for the production of recombinant proteins proliferate spontaneously at a high rate. In many types of cultivation systems these cells still keep growing after having reached the desired cell density. Further proliferation in batch cultures leads to cell death as a consequence of nutrient and oxygen depletion as well as to accumulation of lactate and toxic products. Consequently, in many technical processes, the surplus of cells is removed.We have established cell lines in which proliferation is controlled by a physiological regulator, IRF-1. IRF-1 (Interferon Regulatory Factor 1) is a transcriptional activator and acts as a tumor suppressor. Constitutive overexpression of recombinant IRF-1 leads to inhibition of cell growth. The extent of this growth arrest depends on the intracellular concentration of active IRF-1. To allow IRF-1 expression in various mammalian cells a system for conditional IRF-1 activation has been established. A fusion protein composed of IRF-1 and the hormone binding domain of the human estrogen receptor, was used. This system allows to control gradually the growth of several mammalian cell lines by adjusting the intracellular concentration of active IRF-1 via estradiol in the medium. We have evaluated BHK-21 cells with respect to IRF-1 mediated cell growth inhibition and expression of two secreted proteins. Whereas the productivity of proliferation inhibited cells with respect to constitutively transcribed IgG genes is reduced, productivity of another secreted protein which is controlled by an IRF-1 inducible promoter is strongly enhanced under these conditions.

Reversible Silencing of Cytomegalovirus Genomes by Type I Interferon Governs Virus Latency

Herpesviruses establish a lifelong latent infection posing the risk for virus reactivation and disease. In cytomegalovirus infection, expression of the major immediate early (IE) genes is a critical checkpoint, driving the lytic replication cycle upon primary infection or reactivation from latency. While it is known that type I interferon (IFN) limits lytic CMV replication, its role in latency and reactivation has not been explored. In the model of mouse CMV infection, we show here that IFNβ blocks mouse CMV replication at the level of IE transcription in IFN-responding endothelial cells and fibroblasts. The IFN-mediated inhibition of IE genes was entirely reversible, arguing that the IFN-effect may be consistent with viral latency. Importantly, the response to IFNβ is stochastic, and MCMV IE transcription and replication were repressed only in IFN-responsive cells, while the IFN-unresponsive cells remained permissive for lytic MCMV infection. IFN blocked the viral lytic replication cycle by upregulating the nuclear domain 10 (ND10) components, PML, Sp100 and Daxx, and their knockdown by shRNA rescued viral replication in the presence of IFNβ. Finally, IFNβ prevented MCMV reactivation from endothelial cells derived from latently infected mice, validating our results in a biologically relevant setting. Therefore, our data do not only define for the first time the molecular mechanism of IFN-mediated control of CMV infection, but also indicate that the reversible inhibition of the virus lytic cycle by IFNβ is consistent with the establishment of CMV latency.

Interferon regulatory factor-1 protects from fatal neurotropic infection with vesicular stomatitis virus by specific inhibition of viral replication in neurons.

The innate immune system protects cells against invading viral pathogens by the auto- and paracrine action of type I interferon (IFN). In addition, the interferon regulatory factor (IRF)-1 can induce alternative intrinsic antiviral responses. Although both, type I IFN and IRF-1 mediate their antiviral action by inducing overlapping subsets of IFN stimulated genes, the functional role of this alternative antiviral action of IRF-1 in context of viral infections in vivo remains unknown. Here, we report that IRF-1 is essential to counteract the neuropathology of vesicular stomatitis virus (VSV). IFN- and IRF-1-dependent antiviral responses act sequentially to create a layered antiviral protection program against VSV infections. Upon intranasal infection, VSV is cleared in the presence or absence of IRF-1 in peripheral organs, but IRF-1−/− mice continue to propagate the virus in the brain and succumb. Although rapid IFN induction leads to a decline in VSV titers early on, viral replication is re-enforced in the brains of IRF-1−/− mice. While IFN provides short-term protection, IRF-1 is induced with delayed kinetics and controls viral replication at later stages of infection. IRF-1 has no influence on viral entry but inhibits viral replication in neurons and viral spread through the CNS, which leads to fatal inflammatory responses in the CNS. These data support a temporal, non-redundant antiviral function of type I IFN and IRF-1, the latter playing a crucial role in late time points of VSV infection in the brain.

Proliferation control of mammalian cells by the tumor suppressor IRF-1

We have attempted to establish a system in which cell proliferation is controlled by a physiological regulator. Interferon regulatory factor 1 (IRF-1) is a transcription factor that recognizes a sequence which is present in the interferon-β promoter as well as in the promoters of interferon-inducible genes. IRF-1 acts as a tumor suppressor. Constitutive overexpression of recombinant IRF-1 leads to inhibition of cell growth. The extent of this growth arrest depends on the intracellular concentration of IRF-1. In order to allow IRF-1 expression in various mammalian cells we have established two different systems for conditional IRF-1 transcription and activation, respectively. In one case, an inducible promoter, in the other case a fusion protein composed of IRF-1 and the hormone-binding domain of the human estrogen receptor was used. Both systems allow to control gradually the growth of mammalian cell lines by adjusting the intracellular concentration of IRF-1 via estradiol or tetracycline in the medium. Despite the activity of IRF-1 as an antiproliferative agent the expression of certain proteins is retained. Moreover, expression of genes which are controlled by IRF-1 responsive promoters is enhanced.

Enhanced in Vivo Efficacy of a Type I Interferon Superagonist with Extended Plasma Half-life in a Mouse Model of Multiple Sclerosis

Background: IFNβ constitutes an approved drug to treat multiple sclerosis (MS), but it has limited efficacy. Results: A modified human IFN variant, which exhibits both superagonist properties and 10-fold increased lifespan, outperforms IFNβ in an animal MS model. Conclusion: This drug candidate has potential to supersede IFNβ for the treatment of MS. Significance: Protein engineering allows development of more effective drugs to treat autoimmune diseases. IFNβ is a common therapeutic option to treat multiple sclerosis. It is unique among the family of type I IFNs in that it binds to the interferon receptors with high affinity, conferring exceptional biological properties. We have previously reported the generation of an interferon superagonist (dubbed YNSα8) that is built on the backbone of a low affinity IFNα but modified to exhibit higher receptor affinity than even for IFNβ. Here, YNSα8 was fused with a 600-residue hydrophilic, unstructured N-terminal polypeptide chain comprising proline, alanine, and serine (PAS) to prolong its plasma half-life via “PASylation.” PAS-YNSα8 exhibited a 10-fold increased half-life in both pharmacodynamic and pharmacokinetic assays in a transgenic mouse model harboring the human receptors, notably without any detectable loss in biological potency or bioavailability. This long-lived superagonist conferred significantly improved protection from MOG35–55-induced experimental autoimmune encephalomyelitis compared with IFNβ, despite being injected with a 4-fold less frequency and at an overall 16-fold lower dosage. These data were corroborated by FACS measurements showing a decrease of CD11b+/CD45hi myeloid lineage cells detectable in the CNS, as well as a decrease in IBA+ cells in spinal cord sections determined by immunohistochemistry for PAS-YNSα8-treated animals. Importantly, PAS-YNSα8 did not induce antibodies upon repeated administration, and its biological efficacy remained unchanged after 21 days of treatment. A striking correlation between increased levels of CD274 (PD-L1) transcripts from spleen-derived CD4+ cells and improved clinical response to autoimmune encephalomyelitis was observed, indicating that, at least in this mouse model of multiple sclerosis, CD274 may serve as a biomarker to predict the effectiveness of IFN therapy to treat this complex disease.