The Jak/STAT pathway in model organisms: emerging roles in cell movement

The JAK/STAT pathway was originally identified in mammals. Studies of this pathway in the mouse have revealed that JAK/STAT signaling plays a central role during hematopoeisis and other developmental processes. The role of JAK/STAT signaling in blood appears to be conserved throughout evolution, as it is also required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and zebrafish have shown that the JAK/STAT pathway is also required in an unusually broad set of developmental decisions, including cell proliferation, cell fate determination, cell migration, planar polarity, convergent extension, and immunity. There is increasing evidence that the versatility of this pathway relies on its cooperation with other signal transduction pathways. In this review, we discuss the components of the JAK/STAT pathway in model organisms and what is known about its requirement in cellular and developmental processes. In particular, we emphasize recent insights into the role that this pathway plays in the control of cell movement.     

A new trick for Cyclin-Cdk: activation of STAT

New work in Drosophila demonstrates that cdk4 loss causes phenotypes similar to the loss of JAK/STAT pathway components. Cdk4 overexpression can bypass requirements for JAK but not STAT. These results demonstrate a new function for Cdk4 and a new mode of STAT activation.     

A novel functional activator of the Drosophila JAK/STAT pathway, unpaired2, is revealed by an in vivo reporter of pathway activation

Striking similarities continue to emerge between the mammalian and Drosophila JAK/STAT signaling pathway. However, until now there has not been the ability to monitor global pathway activity during development. We have generated a transgenic animal with a JAK/STAT responsive reporter gene that can be used to monitor pathway activation in whole Drosophila embryos. Expression of the lacZ reporter regulated by STAT92E binding sites can be detected throughout embryogenesis, and is responsive to the Janus Kinase hopscotch and the ligand upd. The system has enabled us to identify the effect of a predicted gene related to upd, designated upd2, whose expression initiates during germ band extension. The stimulatory effect of upd2 on the JAK/STAT reporter can also be demonstrated in Drosophila tissue culture cells. This reporter system will benefit future investigations of JAK/STAT signaling modulators both in whole animals and tissue culture.     

Not4 enhances JAK/STAT pathway-dependent gene expression in Drosophila and in human cells

The JAK/STAT pathway is essential for organogenesis, innate immunity, and stress responses in Drosophila melanogaster. The JAK/STAT pathway and its associated regulators have been highly conserved in evolution from flies to humans. We have used a genome-wide RNAi screen in Drosophila S2 cells to identify regulators of the JAK/STAT pathway, and here we report the characterization of Not4 as a positive regulator of the JAK/STAT pathway. Overexpression of Not4 enhanced Stat92E-mediated gene responses in vitro and in vivo in Drosophila. Specifically, Not4 increased Stat92E-mediated reporter gene activation in S2 cells; and in flies, Not4 overexpression resulted in an 8-fold increase in Turandot M (TotM) and in a 4-fold increase in Turandot A (TotA) stress gene activation when compared to wild-type flies. Drosophila Not4 is structurally related to human CNOT4, which was found to regulate interferon-γ- and interleukin-4-induced STAT-mediated gene responses in human HeLa cells. Not4 was found to coimmunoprecipitate with Stat92E but not to affect tyrosine phosphorylation of Stat92E in Drosophila cells. However, Not4 is required for binding of Stat92E to its DNA recognition sequence in the TotM gene promoter. In summary, Not4/CNOT4 is a novel positive regulator of the JAK/STAT pathway in Drosophila and in humans.     

Characterisation of Upd2, a Drosophila JAK/STAT pathway ligand

The characterisation of ligands that activate the JAK/STAT pathway has the potential to throw light onto a comparatively poorly understood aspect of this important signal transduction cascade. Here, we describe our analysis of the only invertebrate JAK/STAT pathway ligands identified to date, the Drosophila unpaired-like family. We show that upd2 is expressed in a pattern essentially identical to that of upd and demonstrate that the proteins encoded by this region activate JAK/STAT pathway signalling. Mutational analysis demonstrates a mutual semi-redundancy that can be visualised in multiple tissues known to require JAK/STAT signalling. In order to better characterise the in vivo function of these ligands, we developed a reporter based on a natural JAK/STAT pathway responsive enhancer and show that ectopic upd2 expression can effectively activate the JAK/STAT pathway. While both Upd and Upd2 are secreted JAK/STAT pathway agonists, tissue culture assays show that the signal-sequences of Upd and Upd2 confer distinct properties, with Upd associated primarily with the extracellular matrix and Upd2 secreted into the media. The differing biophysical characteristics identified for Upd-like molecules have implications for their function in vivo and adds another aspect to our understanding of cytokine signalling in Drosophila.     

Identification of Drosophila genes modulating Janus kinase/signal transducer and activator of transcription signal transduction

The JAK/STAT pathway was first identified in mammals as a signaling mechanism central to hematopoiesis and has since been shown to exert a wide range of pleiotropic effects on multiple developmental processes. Its inappropriate activation is also implicated in the development of numerous human malignancies, especially those derived from hematopoietic lineages. The JAK/STAT signaling cascade has been conserved through evolution and although the pathway identified in Drosophila has been closely examined, the full complement of genes required to correctly transduce signaling in vivo remains to be identified. We have used a dosage-sensitive dominant eye overgrowth phenotype caused by ectopic activation of the JAK/STAT pathway to screen 2267 independent, newly generated mutagenic P-element insertions. After multiple rounds of retesting, 23 interacting loci that represent genes not previously known to interact with JAK/STAT signaling have been identified. Analysis of these genes has identified three signal transduction pathways, seven potential components of the pathway itself, and six putative downstream pathway target genes. The use of forward genetics to identify loci and reverse genetic approaches to characterize them has allowed us to assemble a collection of genes whose products represent novel components and regulators of this important signal transduction cascade.     

GFP reporters detect the activation of the Drosophila JAK/STAT pathway in vivo

JAK/STAT signaling is essential for a wide range of developmental processes in Drosophila melanogaster. The mechanism by which the JAK/STAT pathway contributes to these processes has been the subject of recent investigation. However, a reporter that reflects activity of the JAK/STAT pathway in all Drosophila tissues has not yet been developed. By placing a fragment of the Stat92E target gene Socs36E, which contains at least two putative Stat92E binding sites, upstream of GFP, we generated three constructs that can be used to monitor JAK/STAT pathway activity in vivo. These constructs differ by the number of Stat92E binding sites and the stability of GFP. The 2XSTAT92E-GFP and 10XSTAT92E-GFP constructs contain 2 and 10 Stat92E binding sites, respectively, driving expression of enhanced GFP, while 10XSTAT92E-DGFP drives expression of destabilized GFP. We show that these reporters are expressed in the embryo in an overlapping pattern with Stat92E protein and in tissues where JAK/STAT signaling is required. In addition, these reporters accurately reflect JAK/STAT pathway activity at larval stages, as their expression pattern overlaps that of the activating ligand unpaired in imaginal discs. Moreover, the STAT92E-GFP reporters are activated by ectopic JAK/STAT signaling. STAT92E-GFP fluorescence is increased in response to ectopic upd in the larval eye disc and mis-expression of the JAK kinase hopscotch in the adult fat body. Lastly, these reporters are specifically activated by Stat92E, as STAT92E-GFP reporter expression is lost cell-autonomously in stat92E homozygous mutant tissue. In sum, we have generated in vivo GFP reporters that accurately reflect JAK/STAT pathway activation in a variety of tissues. These reporters are valuable tools to further investigate and understand the role of JAK/STAT signaling in Drosophila.     

The Polyhomeotic protein induces hyperplastic tissue overgrowth through the activation of the JAK/STAT pathway

Epigenetic mechanisms controlling cellular proliferation are essential to animal development. Moreover, altered levels of expression of the epigenetic regulator proteins are associated with the development and progression of human diseases like cancer. We have studied the effects of high levels of Polyhomeotic (PH) protein, a member of the Polycomb Group (PcG), during the proliferation of the imaginal discs in Drosophila. Over expression of PH protein causes induction of proliferation, accompanied with induction of JNK-dependent apoptosis. As a result, massive hyperplastic overgrowth is produced and the corresponding differentiated tissues show phenotypes related with mis-regulation of homeotic gene expression. We have found that high levels of PH up-regulate the JAK/STAT pathway through the de-repression of Unpaired (UPD), the extracellular ligand of the Drosophila JAK/STAT signalling cascade. Moreover, inactivation of the JAK/STAT pathway in the presence of a large amount of PH protein greatly reduces the tissue overgrowth, demonstrating a functional role of JAK/STAT in PH-induced hyperplasia. Finally, we have observed that decapentaplegic and d-myc, two growth genes and putative targets of the JAK/STAT pathway, are also over expressed in the PH-induced tumors. We propose that during normal development, the PcG proteins act to maintain inactive the JAK/STAT pathway. Upon cellular stress, changes in the levels of PcG proteins expression are induced and JAK/STAT is activated leading to tumor development. Our results show a functional relationship between the PcG gene expression and the JAK/STAT pathway, both of which are found to be perturbed in tumorigenesis.     

Signaling role of hemocytes in Drosophila JAK/STAT-dependent response to septic injury

To characterize the features of JAK/STAT signaling in Drosophila immune response, we have identified totA as a gene that is regulated by the JAK/STAT pathway in response to septic injury. We show that septic injury triggers the hemocyte-specific expression of upd3, a gene encoding a novel Upd-like cytokine that is necessary for the JAK/STAT-dependent activation of totA in the Drosophila counterpart of the mammalian liver, the fat body. In addition, we demonstrate that totA activation also requires the NF-KB-like Relish pathway, indicating that fat body cells integrate the activity of NF-KB and JAK/STAT signaling pathways upon immune response. This study reveals that, in addition to the pattern recognition receptor-mediated NF-KB-dependent immune response, Drosophila undergoes a complex systemic response that is mediated by the production of cytokines in blood cells, a process that is similar to the acute phase response in mammals.     

The fertile field of Drosophila Jak/STAT signalling

The JAK/STAT pathway plays important roles in vertebrate and invertebrate development. The recent cloning and characterisation of the receptor in Drosophila shows that the pathway is conserved across phyla. In this review we describe current knowledge of the pathway and use genome data to discuss what elements are present in Drosophila. We also summarise recent work describing the involvement of the JAK/STAT pathway in oogenesis and spermatogenesis. Interestingly, the JAK/STAT pathway maintains the niche required for germline stem cell maintenance in the testis, providing the first molecular characterisation of a stem cell niche. Drosophila's streamlined pathway offers a simple model to find new elements and analyse the function of existing ones.     

Regulation of circadian behavioral output via a MicroRNA-JAK/STAT circuit

Although molecular components of the circadian clock are known, mechanisms that transmit signals from the clock and produce rhythmic behavior are poorly understood. We find that the microRNA miR-279 regulates the JAK/STAT pathway to drive rest:activity rhythms in Drosophila. Overexpression of microRNA miR-279 or miR-279 deletion attenuates rest:activity rhythms. Oscillations of the clock protein PERIOD are normal in pacemaker neurons lacking miR-279, suggesting that miR-279 acts downstream of the clock. We identify the JAK/STAT ligand, Upd, as a target of miR-279 and show that knockdown of Upd rescues the behavioral phenotype of miR-279 mutants. Manipulations of the JAK/STAT pathway also disrupt circadian rhythms. In addition, central clock neurons project in the vicinity of Upd-expressing neurons, providing a possible physical connection by which the central clock could regulate JAK/STAT signaling to control rest:activity rhythms.     

Novel level of signalling control in the JAK/STAT pathway revealed by in situ visualisation of protein-protein interaction during Drosophila development

It is commonly accepted that activation of most signalling pathways is induced by ligand receptor dimerisation. This belief has been challenged for some vertebrate cytokine receptors of the JAK/STAT pathway. Here we study whether DOME, the Drosophila receptor of the JAK/STAT pathway, can dimerise and if the dimerisation is ligand-dependent. To analyse DOME homo-dimerisation, we have applied a beta-gal complementation technique that allows the detection of protein interactions in situ. This technique has been used previously in cell culture but this is the first time that it has been applied to whole embryos. We show that this technique, which we rename betalue-betalau technique, can be used to detect DOME homo-dimerisation in Drosophila developing embryos. Despite DOME being ubiquitously expressed, dimerisation is developmentally regulated. We investigate the state of DOME dimerisation in the presence or absence of ligand and show that DOME dimerisation is not ligand-induced, indicating that ligand independent cytokine receptor dimerisation is a conserved feature across phyla. We have further analysed the functional significance of ligand-independent receptor dimerisation by comparing the effects of ectopic ligand expression in cells in which the receptor is, or is not, dimerised. We show that ligand expression can only activate STAT downstream targets or affect embryo development in cells in which the receptor is dimerised. These results suggest a model in which ligand-independent dimerisation of the JAK/STAT receptor confers cells with competence to activate the pathway prior to ligand reception. Thus, competence to induce the JAK/STAT signalling pathway in Drosophila can be regulated by controlling receptor dimerisation prior to ligand binding. These results reveal a novel level of JAK/STAT signalling regulation that could also apply to vertebrates.     

Paracrine signaling through the JAK/STAT pathway activates invasive behavior of ovarian epithelial cells in Drosophila.

The JAK/STAT signaling pathway, renowned for its effects on cell proliferation and survival, is constitutively active in various human cancers, including ovarian. We have found that JAK and STAT are required to convert the border cells in the Drosophila ovary from stationary, epithelial cells to migratory, invasive cells. The ligand for this pathway, Unpaired (UPD), is expressed by two central cells within the migratory cell cluster. Mutations in upd or jak cause defects in migration and a reduction in the number of cells recruited to the cluster. Ectopic expression of either UPD or JAK is sufficient to induce extra epithelial cells to migrate. Thus, a localized signal activates the JAK/STAT pathway in neighboring epithelial cells, causing them to become invasive.