The cytokine-activated Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway plays an important role in the control of a wide variety of biological processes. When misregulated, JAK/STAT signaling is associated with various human diseases, such as immune disorders and tumorigenesis. To gain insights into the mechanisms by which JAK/STAT signaling participates in these diverse biological responses, we carried out a genome-wide RNA interference (RNAi) screen in cultured Drosophila cells. We identified 121 genes whose double-stranded RNA (dsRNA)-mediated knockdowns affected STAT92E activity. Of the 29 positive regulators, 13 are required for the tyrosine phosphorylation of STAT92E. Furthermore, we found that the Drosophila homologs of RanBP3 and RanBP10 are negative regulators of JAK/STAT signaling through their control of nucleocytoplasmic transport of STAT92E. In addition, we identified a key negative regulator of Drosophila JAK/STAT signaling, protein tyrosine phosphatase PTP61F, and showed that it is a transcriptional target of JAK/STAT signaling, thus revealing a novel negative feedback loop. Our study has uncovered many uncharacterized genes required for different steps of the JAK/STAT signaling pathway.

Innate immune responses are mediated by the activation of various signaling processes. Here, we describe our current knowledge on Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling in the Drosophila immune response. First, we briefly introduce the main effectors involved in the humoral and cellular responses, such as anti-bacterial peptides and hemocytes. Second, we describe the canonical JAK/STAT-signaling pathway, as established from extensive studies in mammalian systems, and we introduce the Drosophila components of the JAK/STAT pathway, as discovered from studies on embryonic development. Third, we describe the various roles of JAK/STAT signaling in both humoral and cellular responses. We present the JAK/STAT-dependent humoral factors, such as the thioester-containing proteins and the Tot peptides, produced by the fat body in response to septic injury. We also discuss the possible involvement of the JAK/STAT pathway in cellular responses, including hemocyte proliferation and differentiation. Finally, we present how cytokines, such as Upd3, might contribute to the integration of the immune responses at the organism level by orchestrating the response of various immune cells and organs, such as fat body, hemocytes, and lymph glands.

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 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.

The JAK/STAT signal transduction pathway has been conserved throughout evolution such that true structural and functional homologues of components originally identified in vertebrate systems are also present in the model genetic system Drosophila melanogaster. In addition to roles during larval hematopoiesis reminiscent of the requirement for this pathway in mammalian systems, the JAK/STAT pathway in Drosophila is also involved in a number of other developmental events. Recent data has demonstrated further roles for the JAK/STAT pathway in the establishment of sexual identity via the early embryonic expression of Sex lethal, the segmentation of the embryo via the control of pair rule genes including even skipped and the establishment of polarity within the adult compound eye via a mechanism that includes the four jointed gene. Use of the powerful genetics in the model organism Drosophila may identify new components of the JAK/STAT pathway, define new roles for this pathway, and provide insights into the function of this signal transduction system. Here we review the roles of STAT and its associated signaling pathway during both embryonic and adult stages of Drosophila development and discuss future prospects for the identification and characterization of novel pathway components and targets. Oncogene (2000).

The JAK/STAT signaling pathway is required for many processes including cytokine signaling, hematopoiesis, gliagenesis, and Drosophila segmentation. In this report we present evidence demonstrating that the JAK/STAT pathway is also central to the establishment of planar polarity during Drosophila eye development. We show that a localized source of the pathway ligand, Unpaired, is present at the midline of the developing eye, which is capable of activating the JAK/STAT pathway over long distances. A gradient of JAK/STAT activity across the DV axis of the eye regulates ommatidial polarity via an unidentified second signal. Additionally, localized Unpaired influences the position of the equator via repression of mirror.

In vertebrates, many cytokines and growth factors have been identified as activators of the JAK/STAT signaling pathway. In Drosophila, JAK and STAT molecules have been isolated, but no ligands or receptors capable of activating the pathway have been described. We have characterized the unpaired (upd) gene, which displays the same distinctive embryonic mutant defects as mutations in the Drosophila JAK (hopscotch) and STAT (stat92E) genes. Upd is a secreted protein, associated with the extracellular matrix, that activates the JAK pathway. We propose that Upd is a ligand that relies on JAK signaling to stimulate transcription of pair-rule genes in a segmentally restricted manner in the early Drosophila embryo.

Recent studies in Drosophila have identified a single JAK and a single STAT protein. Genetic and biochemical analyses reveal that these two proteins operate in the same signal transduction pathway. Phenotypic analyses of JAK and STAT mutants implicate this pathway in a number of developmental decisions, in particular the regulation of pair-rule genes and fly hematopoiesis.

We have identified a putative Drosophila STAT protein named Marelle that exhibits mutant phenotypes identical to mutations in the Hopscotch/JAK kinase. We show that a reduction in the amount of marelle gene activity suppresses the phenotype associated with a gain-of-function mutation in hopscotch and enhances the phenotype associated with a weak hopscotch mutation. We propose that Hopscotch activates Marelle to regulate transcription of target genes such as the pair rule gene even-skipped. Our results demonstrate the existence of an invertebrate JAK/STAT system.

In mammals, many cytokines and growth factors stimulate members of the Janus kinase (JAK) family to transduce signals for the proliferation and differentiation of various cell types, particularly in hematopoietic lineages. Mutations in the Drosophila hopscotch (hop) gene, which encodes a JAK, also cause proliferative defects. Loss-of-function alleles result in lethality and underproliferation of diploid tissues of the larva. A dominant gain-of-function allele, Tumorous-lethal (hopTum-l), leads to formation of melanotic tumors and hypertrophy of the larval lymph glands, the hematopoietic organs. We show that a single amino acid change in Hop is associated with the hopTum-l mutation. Overexpression of either wild-type hop or hopTum-l in the larval lymph glands causes melanotic tumors and lymph gland hypertrophy indistinguishable from the original hopTum-l mutation. In addition, overexpression of Hop in other tissues of the larva leads to pattern defects in the adult or to lethality. Finally, overexpression of either hop or hopTum-l in Drosophila cell culture results in tyrosine phosphorylation of Hop protein. However, overexpression of hopTum-l results in greater phosphorylation than overexpression of the wild-type. We conclude that hopTum-l encodes a hyperactive Hop kinase and that overactivity of Hop in lymph glands causes malignant neoplasia of Drosophila blood cells.

We describe the characterization of the Drosophila gene, hopscotch (hop), which is required maternally for the establishment of the normal array of embryonic segments. In hop embryos, although expression of the gap genes appears normal, there are defects in the expression patterns of the pair-rule genes even-skipped, runt, and fushi tarazu, as well as the segment-polarity genes engrailed and wingless. We demonstrate that the effect of hop on the expression of these genes is stripe-specific. The hop gene encodes a putative nonreceptor tyrosine kinase of the Janus kinase family, based on an internal duplication of the catalytic domain. We present a model in which the Hop tyrosine kinase is involved in the control of pair-rule gene transcription in a stripe-specific manner. Our results provide the first evidence for stripe-specific regulation of pair-rule genes by a tyrosine kinase.