Tyrosine kinase 2 - Surveillant of tumours and bona fide oncogene

Tyrosine kinase 2 (TYK2) is a member of the Janus kinase (JAK) family, which transduces cytokine and growth factor signalling. Analysis of TYK2 loss-of-function revealed its important role in immunity to infection, (auto-) immunity and (auto-) inflammation. TYK2-deficient patients unravelled high similarity between mice and men with respect to cellular signalling functions and basic immunology. Genome-wide association studies link TYK2 to several autoimmune and inflammatory diseases as well as carcinogenesis. Due to its cytokine signalling functions TYK2 was found to be essential in tumour surveillance. Lately TYK2 activating mutants and fusion proteins were detected in patients diagnosed with leukaemic diseases suggesting that TYK2 is a potent oncogene. Here we review the cell intrinsic and extrinsic functions of TYK2 in the characteristics preventing and enabling carcinogenesis. In addition we describe an unexpected function of kinase-inactive TYK2 in tumour rejection.

Conditional ablation of TYK2 in immunity to viral infection and tumor surveillance

Tyrosine kinase 2 (TYK2) has a pivotal role in immunity to infection and tumor surveillance. It is associated with several cytokine receptor chains including type I interferon (IFN) receptor 1 (IFNAR1), interleukin- (IL-) 12 receptor beta 1 (IL-12Rb1) and IL-10R2. We have generated a mouse with a conditional Tyk2 null allele and proved integrity of the conditional Tyk2 locus. TYK2 was successfully removed by the use of ubiquitous and tissue-specific Cre-expressing mouse strains. Myeloid TYK2 was found to critically contribute to the defense against murine cytomegalovirus. Ubiquitous TYK2 ablation severely impaired tumor immunosurveillance, while deletion in myeloid, dendritic or T cells alone showed no effect. The conditional Tyk2 mouse strain will be instrumental to further dissect TYK2 functions in infection, inflammation and cancer.

Lactotransferrin-Cre reporter mice trace neutrophils, monocytes/macrophages and distinct subtypes of dendritic cells

Considerable effort has been expended to identify genes that account for myeloid lineage commitment and development. However, currently available non-invasive mouse models utilize myeloid-specific reporters that are significantly expressed in hematopoietic stem cells as well as lymphoid compartments. Here, we describe a myeloid-specific marker that is not shared by any other lineage. We show that lactotransferrin mRNA is expressed by Gr-1(+)/CD11b(+) cells in the bone marrow, as opposed to hematopoietic stem cells or any peripheral cell population. To follow the progeny of lactotransferrin-expressing bone marrow cells, we generated a mouse model in which a reporter gene is irreversibly activated from the lactotransferrin-promoter. We found that lactotransferrin-reporter labels a majority of neutrophils, monocytes, macrophages and distinct subtypes of dendritic cells, while excluding T, B, natural killer cells, interferon-producing killer dendritic cells, plasmacytoid dendritic cells, erythrocytes and eosinophils. Lactotransferrin-reporter(-) bone marrow cells retain lymphoid, erythroid and long-term repopulating potential, while lactotransferrin-reporter(+) bone marrow cells confer only myeloid, but not lymphoid potential. We conclude that lactotransferrin represents a late stage differentiation marker of neutrophils, macrophages and distinct subtypes of dendritic cells.

Inducible, dose-adjustable and time-restricted reconstitution of STAT1 deficiency in vivo

Signal transducer and activator of transcription (STAT) 1 is a key player in interferon (IFN) signaling, essential in mediating host defense against viruses and other pathogens. STAT1 levels are tightly regulated and loss- or gain-of-function mutations in mice and men lead to severe diseases. We have generated a doxycycline (dox) -inducible, FLAG-tagged Stat1 expression system in mice lacking endogenous STAT1 (i.e. Stat1^ind mice). We show that STAT1 expression depends on the time and dose of dox treatment in primary cells and a variety of organs isolated from Stat1^ind mice. In bone marrow-derived macrophages, a fraction of the amount of STAT1 present in WT cells is sufficient for full expression of IFN-induced genes. Dox-induced STAT1 established protection against virus infections in primary cells and mice. The availability of the Stat1^ind mouse model will enable an examination of the consequences of variable amounts of STAT1. The model will also permit the study of STAT1 dose-dependent and reversible functions as well as of STAT1's contributions to the development, progression and resolution of disease.

TYK2 kinase activity is required for functional type I interferon responses in vivo

Tyrosine kinase 2 (TYK2) is a member of the Janus kinase (JAK) family and is involved in cytokine signalling. In vitro analyses suggest that TYK2 also has kinase-independent, i.e., non-canonical, functions. We have generated gene-targeted mice harbouring a mutation in the ATP-binding pocket of the kinase domain. The Tyk2 kinase-inactive (Tyk2^K923E) mice are viable and show no gross abnormalities. We show that kinase-active TYK2 is required for full-fledged type I interferon- (IFN) induced activation of the transcription factors STAT1-4 and for the in vivo antiviral defence against viruses primarily controlled through type I IFN actions. In addition, TYK2 kinase activity was found to be required for the protein’s stability. An inhibitory function was only observed upon over-expression of TYK2^K923Ein vitro. Tyk2^K923E mice represent the first model for studying the kinase-independent function of a JAK in vivo and for assessing the consequences of side effects of JAK inhibitors.

Multifaceted Antiviral Actions of Interferon-stimulated Gene Products

Interferons (IFNs) are extremely powerful cytokines for the host defence against viral infections. Binding of IFNs to their receptors activates the JAK/STAT signalling pathway with the Janus kinases JAK1, 2 and TYK2 and the signal transducer and activators of transcription (STAT) 1 and STAT2. Depending on the cellular setting, additional STATs (STAT3-6) and additional signalling pathways are activated. The actions of IFNs on infected cells and the surrounding tissue are mediated by the induction of several hundred IFN-stimulated genes (ISGs). Since the cloning of the first ISGs, considerable progress has been made in describing antiviral effector proteins and their many modes of action. Effector proteins individually target distinct steps in the viral life cycle, including blocking virus entry, inhibition of viral transcription and translation, modification of viral nucleic acids and proteins and, interference with virus assembly and budding. Novel pathways of viral inhibition are constantly being elucidated and, additionally, unanticipated functions of known antiviral effector proteins are discovered. Herein, we outline IFN-induced antiviral pathways and review recent developments in this fascinating area of research.

A comparative proteome analysis links tyrosine kinase 2 (Tyk2) to the regulation of cellular glucose and lipid metabolism in response to poly(I:C)

Tyrosine kinase 2 (Tyk2) is an integral part of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway which relays intracellular signals of various cytokines. Tyk2 crucially contributes to host defense mechanisms against microbial pathogens and to tumor surveillance but also facilitates immune pathologies. Here we investigated the impact of Tyk2 on the macrophage proteome using the synthetic double-stranded RNA analog polyinosinic acid-polycytidylic acid (poly(I:C)) as a mimicry of viral infections. By means of 2D-DIGE in connection with PMF obtained by MALDI-MS and sequence tag determination by MS/MS we unambiguously identified eighteen protein spots corresponding to sixteen distinct proteins that are regulated by poly(I:C) and differentially expressed between wildtype (WT) and Tyk2-deficient macrophages. The majority of these proteins are functionally assigned to cellular immune responses and to metabolism. We show for selected metabolic enzymes, i.e. triosephosphate isomerase (TIM), ATP-citrate synthase (ACLY) and long-chain-fatty-acid-CoA ligase 4 (ACSL4), that Tyk2 affects protein expression transcriptionally and post-transcriptionally. We furthermore confirm the involvement of Tyk2 in the regulation of lipid and carbohydrate metabolism at the level of metabolites. Taken together, our results provide new evidence for important functions of Tyk2 at the molecular interface between innate immunity and cellular metabolism.

A time- and dose-dependent STAT1 expression system

Background

The signal transducer and activator of transcription (STAT) family of transcription factors mediates a variety of cytokine dependent gene regulations. STAT1 has been mainly characterized by its role in interferon (IFN) type I and II signaling and STAT1 deficiency leads to high susceptibility to several pathogens. For fine-tuned analysis of STAT1 function we established a dimerizer-inducible system for STAT1 expression in vitro and in vivo.

Results

The functionality of the dimerizer-induced STAT1 system is demonstrated in vitro in mouse embryonic fibroblasts and embryonic stem cells. We show that this two-vector based system is highly inducible and does not show any STAT1 expression in the absence of the inducer. Reconstitution of STAT1 deficient cells with inducible STAT1 restores IFNγ-mediated gene induction, antiviral responses and STAT1 activation remains dependent on cytokine stimulation. STAT1 expression is induced rapidly upon addition of dimerizer and expression levels can be regulated in a dose-dependent manner. Furthermore we show that in transgenic mice STAT1 can be induced upon stimulation with the dimerizer, although only at low levels.

Conclusion

These results prove that the dimerizer-induced system is a powerful tool for STAT1 analysis in vitro and provide evidence that the system is suitable for the use in transgenic mice. To our knowledge this is the first report for inducible STAT1 expression in a time- and dose-dependent manner.