Plasticity of Drosophila Stat DNA binding shows an evolutionary basis for Stat transcription factor preferences

Interlocking of co-opted developmental gene networks in Drosophila and the evolution of pre-adaptive novelty

The re-use of genes in new organs forms the base of many evolutionary novelties. A well-characterised case is the recruitment of the posterior spiracle gene network to the Drosophila male genitalia. Here we find that this network has also been co-opted to the testis mesoderm where is required for sperm liberation, providing an example of sequentially repeated developmental co-options. Associated to this co-option event, an evolutionary expression novelty appeared, the activation of the posterior segment determinant Engrailed to the anterior A8 segment controlled by common testis and spiracle regulatory elements. Enhancer deletion shows that A8 anterior Engrailed activation is not required for spiracle development but only necessary in the testis. Our study presents an example of pre-adaptive developmental novelty: the activation of the Engrailed transcription factor in the anterior compartment of the A8 segment where, despite having no specific function, opens the possibility of this developmental factor acquiring one. We propose that recently co-opted networks become interlocked, so that any change to the network because of its function in one organ, will be mirrored by other organs even if it provides no selective advantage to them.

Specification of the endocrine primordia controlling insect moulting and metamorphosis by the JAK/STAT signalling pathway

The corpora allata and the prothoracic glands control moulting and metamorphosis in insects. These endocrine glands are specified in the maxillary and labial segments at positions homologous to those forming the trachea in more posterior segments. Glands and trachea can be homeotically transformed into each other suggesting that all three evolved from a metamerically repeated organ that diverged to form glands in the head and respiratory organs in the trunk. While much is known about tracheal specification, there is limited information about corpora allata and prothorathic gland specification. Here we show that the expression of a key regulator of early gland development, the snail gene, is controlled by the Dfd and Scr Hox genes and by the Hedgehog and Wnt signalling pathways that induce localised transcription of upd, the ligand of the JAK/STAT signalling pathway, which lies at the heart of gland specification. Our results show that the same upstream regulators are required for the early gland and tracheal primordia specification, reinforcing the hypothesis that they originated from a segmentally repeated organ present in an ancient arthropod.

Germline sex determination regulates sex-specific signaling between germline stem cells and their niche

Establishing germ cell sexual identity is critical for development of male and female germline stem cells (GSCs) and production of sperm or eggs. Germ cells depend on signals from the somatic gonad to determine sex, but in organisms such as flies, mice, and humans, the sex chromosome genotype of the germ cells is also important for germline sexual development. How somatic signals and germ-cell-intrinsic cues combine to regulate germline sex determination is thus a key question. We find that JAK/STAT signaling in the GSC niche promotes male identity in germ cells, in part by activating the chromatin reader Phf7. Further, we find that JAK/STAT signaling is blocked in XX (female) germ cells through the action of the sex determination gene Sex lethal to preserve female identity. Thus, an important function of germline sexual identity is to control how GSCs respond to signals in their niche environment.

Dnmt3a is downregulated by Stat5a and mediates G0/G1 arrest by suppressing the miR-17-5p/Cdkn1a axis in Jak2V617F cells

Background

Despite of the frequently reported Dnmt3a abormality in classical myeloproliferative neoplasms (cMPNs) patients, few research explores how the Dnmt3a is regulated by Jak2^V617F mutation. In this study, we have investigated how the Dnmt3a is regulated by Jak2^V617F mutation and its effects on downstream signaling pathways in cMPNs.

Methods

Specimens of Jak2^V617F positive cMPN patients and normal controls were collected. Murine BaF3 cell line was used to construct cell models. Dual-Glo luciferase assays and chromatin immunoprecipitation (ChIP)-qPCR were performed to detect the impact of Stat5a on transcription activity of Dnmt3a. Soft agar colony formation assay and cell counting assay were performed to detect cell proliferation. BrdU staining and flow cytometry were used to investigate cell cycle distribution. Western blotting and quantitative reverse-transcription PCR (qPCR) were performed to detect the expression levels of genes.

Results

Firstly, the results of western blotting and qPCR revealed that compared with the control samples, Dnmt3a is downregulated in Jak2^V617F positive samples. Then we explored the mechanism behind it and found that Dnmt3a is a downstream target of Stat5a, the transcription and translation of Dnmt3a is suppressed by the binding of aberrantly activated Stat5a with Dnmt3a promoter in Jak2^V617F positive samples. We further revealed the region approximately 800 bp upstream of the first exon of the Dnmt3a promoter, which includes a gamma-activated sequence (GAS) motif of Stat5a, is the specific site that Stat5a binds to. Soft agar colony formation assay, cell counting assay, and BrdU staining and flow cytometry assay found that Dnmt3a in Jak2^V617F-BaF3 cells significantly affected the cell proliferation capacity and cell cycle distribution by suppressing Cdkn1a via miR-17-5p/Cdkn1a axis and mediated G0/G1 arrest.

Conclusions

Transcription and translation of Dnmt3a is downregulated by the binding of Stat5a with Dnmt3a promoter in Jak2^V617F cells. The GAS motif at promoter of Dnmt3a is the exact site where the Stat5a binds to. Dnmt3a conducted G0/G1 arrest through regulating miR-17-5p/Cdkn1a axis. The axis of Stat5a/Dnmt3a/miR-17-5p/Cdkn1a potentially provides a treatment target for cMPNs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08915-0.

Thermal stress induces tissue damage and a broad shift in regenerative signaling pathways in the honey bee digestive tract

Honey bee colonies in the USA have suffered from increased die-off in the last few years with a complex set of interacting stresses playing a key role. With changing climate, an increase in the frequency of severe weather events, such as heat waves, is anticipated. Understanding how these changes may contribute to stress in honey bees is crucial. Individual honey bees appear to have a high capacity to endure thermal stress. One reason for this high-level endurance is likely their robust heat shock response (HSR), which contributes to thermotolerance at the cellular level. However, less is known about other mechanisms of thermotolerance, especially those operating at the tissue level. To elucidate other determinants of resilience in this species, we used thermal stress coupled with RNAseq and identified broad transcriptional remodeling of a number of key signaling pathways in the honey bee, including those pathways known to be involved in digestive tract regeneration in the fruit fly such as the Hippo and JAK/STAT pathways. We also observed cell death and shedding of epithelial cells, which likely leads to induction of this regenerative transcriptional program. We found that thermal stress affects many of these pathways in other tissues, suggesting a shared program of damage response. This study provides important foundational characterization of the tissue damage response program in this key pollinating species. In addition, our data suggest that a robust regeneration program may also be a critical contributor to thermotolerance at the tissue level, a possibility which warrants further exploration in this and other species.

The vascular niche controls Drosophila hematopoiesis via fibroblast growth factor signaling

In adult mammals, hematopoiesis, the production of blood cells from hematopoietic stem and progenitor cells (HSPCs), is tightly regulated by extrinsic signals from the microenvironment called 'niche'. Bone marrow HSPCs are heterogeneous and controlled by both endosteal and vascular niches. The Drosophila hematopoietic lymph gland is located along the cardiac tube which corresponds to the vascular system. In the lymph gland, the niche called Posterior Signaling Center controls only a subset of the heterogeneous hematopoietic progenitor population indicating that additional signals are necessary. Here we report that the vascular system acts as a second niche to control lymph gland homeostasis. The FGF ligand Branchless produced by vascular cells activates the FGF pathway in hematopoietic progenitors. By regulating intracellular calcium levels, FGF signaling maintains progenitor pools and prevents blood cell differentiation. This study reveals that two niches contribute to the control ofDrosophila blood cell homeostasis through their differential regulation of progenitors.

Multiple Effects of Ascorbic Acid against Chronic Diseases: Updated Evidence from Preclinical and Clinical Studies

Severe disease commonly manifests as a systemic inflammatory process. Inflammation is associated withthe enhanced production of reactive oxygen and nitrogen species and with a marked reduction in the plasma concentrations of protective antioxidant molecules. This imbalance gives rise to oxidative stress, which is greater in patients with more severe conditions such as sepsis, cancer, cardiovascular disease, acute respiratory distress syndrome, and burns. In these patients, oxidative stress can trigger cell, tissue, and organ damage, thus increasing morbidity and mortality. Ascorbic acid (ASC) is a key nutrient thatserves as an antioxidant and a cofactor for numerous enzymatic reactions. However, humans, unlike most mammals, are unable to synthesize it. Consequently, ASC must be obtained through dietary sources, especially fresh fruit and vegetables. The value of administering exogenous micronutrients, to reestablish antioxidant concentrations in patients with severe disease, has been recognized for decades. Despite the suggestion that ASC supplementation may reduce oxidative stress and prevent several chronic conditions, few large, randomized clinical trials have tested it in patients with severe illness. This article reviews the recent literature on the pharmacological profile of ASC and the role of its supplementation in critically ill patients.

Reactive oxygen species-dependent Toll/NF-κB activation in the Drosophila hematopoietic niche confers resistance to wasp parasitism

Hematopoietic stem/progenitor cells in the adult mammalian bone marrow ensure blood cell renewal. Their cellular microenvironment, called 'niche', regulates hematopoiesis both under homeostatic and immune stress conditions. In the Drosophila hematopoietic organ, the lymph gland, the posterior signaling center (PSC) acts as a niche to regulate the hematopoietic response to immune stress such as wasp parasitism. This response relies on the differentiation of lamellocytes, a cryptic cell type, dedicated to pathogen encapsulation and killing. Here, we establish that Toll/NF-κB pathway activation in the PSC in response to wasp parasitism non-cell autonomously induces the lymph gland immune response. Our data further establish a regulatory network where co-activation of Toll/NF-κB and EGFR signaling by ROS levels in the PSC/niche controls lymph gland hematopoiesis under parasitism. Whether a similar regulatory network operates in mammals to control emergency hematopoiesis is an open question.

A Drosophila model of myeloproliferative neoplasm reveals a feed-forward loop in the JAK pathway mediated by p38 MAPK signalling

Myeloproliferative neoplasms (MPNs) of the Philadelphia-negative class comprise polycythaemia vera, essential thrombocythaemia and primary myelofibrosis (PMF). They are associated with aberrant numbers of myeloid lineage cells in the blood, and in the case of overt PMF, with development of myelofibrosis in the bone marrow and failure to produce normal blood cells. These diseases are usually caused by gain-of-function mutations in the kinase JAK2. Here, we use Drosophila to investigate the consequences of activation of the JAK2 orthologue in haematopoiesis. We have identified maturing haemocytes in the lymph gland, the major haematopoietic organ in the fly, as the cell population susceptible to induce hypertrophy upon targeted overexpression of JAK. We show that JAK activates a feed-forward loop, including the cytokine-like ligand Upd3 and its receptor, Domeless, which are required to induce lymph gland hypertrophy. Moreover, we present evidence that p38 MAPK signalling plays a key role in this process by inducing expression of the ligand Upd3. Interestingly, we also show that forced activation of the p38 MAPK pathway in maturing haemocytes suffices to generate hypertrophic organs and the appearance of melanotic tumours. Our results illustrate a novel pro-tumourigenic crosstalk between the p38 MAPK pathway and JAK signalling in a Drosophila model of MPNs. Based on the shared molecular mechanisms underlying MPNs in flies and humans, the interplay between Drosophila JAK and p38 signalling pathways unravelled in this work might have translational relevance for human MPNs.

Summary: Pro-tumourigenic crosstalk occurs between the p38 MAPK pathway and JAK signalling in a Drosophila model of myeloproliferative neoplasm.

Two Independent Functions of Collier/Early B Cell Factor in the Control of Drosophila Blood Cell Homeostasis

Blood cell production in the Drosophila hematopoietic organ, the lymph gland, is controlled by intrinsic factors and extrinsic signals. Initial analysis of Collier/Early B Cell Factor function in the lymph gland revealed the role of the Posterior Signaling Center (PSC) in mounting a dedicated cellular immune response to wasp parasitism. Further, premature blood cell differentiation when PSC specification or signaling was impaired, led to assigning the PSC a role equivalent to the vertebrate hematopoietic niche. We report here that Collier is expressed in a core population of lymph gland progenitors and cell autonomously maintains this population. The PSC contributes to lymph gland homeostasis by regulating blood cell differentiation, rather than by maintaining core progenitors. In addition to PSC signaling, switching off Collier expression in progenitors is required for efficient immune response to parasitism. Our data show that two independent sites of Collier/Early B Cell Factor expression, hematopoietic progenitors and the PSC, achieve control of hematopoiesis.

JAK/STAT and Hox Dynamic Interactions in an Organogenetic Gene Cascade

Organogenesis is controlled by gene networks activated by upstream selector genes. During development the gene network is activated stepwise, with a sequential deployment of successive transcription factors and signalling molecules that modify the interaction of the elements of the network as the organ forms. Very little is known about the steps leading from the early specification of the cells that form the organ primordium to the moment when a robust gene network is in place. Here we study in detail how a Hox protein induces during early embryogenesis a simple organogenetic cascade that matures into a complex gene network through the activation of feedback and feed forward interaction loops. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT signalling pathway. We find that at later stages STAT activity feeds back directly into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on crumbs, a spiracle downstream target gene of Abd-B, we analyze how a modular cis regulatory element integrates the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. We describe how a Hox induced genetic cascade transforms into a robust gene network during organogenesis due to the repeated interaction of Abd-B and one of its targets, the JAK/STAT signalling cascade. Our results show that in this network STAT functions not just as a direct transcription factor, but also acts as a "counter-repressor", uncovering a novel mode for STAT directed transcriptional regulation.

Organogenesis is controlled by gene networks activated by upstream selector genes. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT pathway. We find that at later stages STAT activity feeds back into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on a spiracle downstream target gene of Abd-B, we analyze how its cis regulatory elements integrate the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. Our results also show that the well known transcription factor STAT can control gene expression as a “counter-repressor”, uncovering an alternative novel mode for STAT directed transcriptional regulation.

The epigenetic regulator G9a mediates tolerance to RNA virus infection in Drosophila

Little is known about the tolerance mechanisms that reduce the negative effects of microbial infection on host fitness. Here, we demonstrate that the histone H3 lysine 9 methyltransferase G9a regulates tolerance to virus infection by shaping the response of the evolutionary conserved Jak-Stat pathway in Drosophila. G9a-deficient mutants are more sensitive to RNA virus infection and succumb faster to infection than wild-type controls, which was associated with strongly increased Jak-Stat dependent responses, but not with major differences in viral load. Genetic experiments indicate that hyperactivated Jak-Stat responses are associated with early lethality in virus-infected flies. Our results identify an essential epigenetic mechanism underlying tolerance to virus infection.

Multicellular organisms deploy various strategies to fight microbial infections. Invading pathogens may be eradicated directly by antimicrobial effectors of the immune system. Another strategy consists of increasing the tolerance of the host to infection, for example, by limiting the adverse effects of the immune response. The molecular mechanisms underlying this novel concept remain largely uncharacterized. Here, we demonstrate that the epigenetic regulator G9a mediates tolerance to virus infection in Drosophila. We found that G9a-deficient flies succumb faster than control flies to infection with RNA viruses, but that the viral burden did not significantly differ. Unexpectedly, mutant flies express higher levels of genes that are regulated by the Jak-Stat signaling pathway, which in other studies was found to be important for antiviral defense. Exploiting the genetic toolbox in Drosophila, we demonstrate that Jak-Stat hyperactivation induces early mortality after virus infection. Precise control of immune pathways is essential to ensure efficient immunity, while preventing damage due to excessive immune responses. Our results indicate that G9a, an epigenetic modifier, dampens Jak-Stat responses to prevent immunopathology. Therefore, we propose epigenetic regulation of immunity as a new paradigm for disease tolerance.

PI3K signaling and Stat92E converge to modulate glial responsiveness to axonal injury

Activation of glial cells following axon injury is mediated by a positive feedback loop downstream of the glial phagocytic receptor Draper, allowing the strength of the response to match the severity of injury.

Glial cells are exquisitely sensitive to neuronal injury but mechanisms by which glia establish competence to respond to injury, continuously gauge neuronal health, and rapidly activate reactive responses remain poorly defined. Here, we show glial PI3K signaling in the uninjured brain regulates baseline levels of Draper, a receptor essential for Drosophila glia to sense and respond to axonal injury. After injury, Draper levels are up-regulated through a Stat92E-modulated, injury-responsive enhancer element within the draper gene. Surprisingly, canonical JAK/STAT signaling does not regulate draper expression. Rather, we find injury-induced draper activation is downstream of the Draper/Src42a/Shark/Rac1 engulfment signaling pathway. Thus, PI3K signaling and Stat92E are critical in vivo regulators of glial responsiveness to axonal injury. We provide evidence for a positive auto-regulatory mechanism whereby signaling through the injury-responsive Draper receptor leads to Stat92E-dependent, transcriptional activation of the draper gene. We propose that Drosophila glia use this auto-regulatory loop as a mechanism to adjust their reactive state following injury.

Acute injuries of the central nervous system (CNS) trigger a robust reaction from glial cells—a non-neuronal population of cells that regulate and support neural development and physiology. Although this process occurs after all types of CNS trauma in mammals, how it is activated and its precise role in recovery remain poorly understood. Using the fruit fly Drosophila melanogaster as a model, we previously identified a cell surface receptor called Draper, which is required for the activation of glia after local axon injury (“axotomy”) and for the removal of degenerating axonal debris by phagocytosis. Here, we show that regulation of Draper protein levels and glial activation through the Draper signaling pathway are mediated by the well-conserved PI3K and signal transducer and activator of transcription (STAT) signaling cascades. We find that STAT transcriptional activity is activated in glia in response to axotomy, and identify an injury-responsive regulatory element within the draper gene that appears to be directly modulated by STAT. Interestingly, the intensity of STAT activity in glial cells after axotomy correlates tightly with the number of local severed axons, indicating that Drosophila glia are able to fine-tune their response to neuronal injury according to its severity. In summary, we propose that the initial phagocytic competence of glia is regulated by setting Draper baseline levels (via PI3K), whereas injury-activated glial phagocytic activity is modulated through a positive feedback loop that requires STAT-dependent activation of draper. We speculate that the level of activation of this cascade is determined by glial cell recognition of Draper ligands present on degenerating axon material, thereby matching the levels of glial reactivity to the amount of injured axonal material.

Gut microbiota of the tick vector Ixodes scapularis modulate colonization of the Lyme disease spirochete

Arthopods such as Ixodes scapularis ticks serve as vectors for many human pathogens. The arthropod gut presents a pivotal microbial entry point and determines pathogen colonization and survival. We show that the gut microbiota of I. scapularis, a major vector of the Lyme disease spirochete Borrelia burgdorferi, influence spirochete colonization of ticks. Perturbing the gut microbiota of larval ticks reduced Borrelia colonization, and dysbiosed larvae displayed decreased expression of the transcription factor signal transducer and activator of transcription (STAT). Diminished STAT expression corresponded to lower expression of peritrophin, a key glycoprotein scaffold of the glycan-rich mucus-like peritrophic matrix (PM) that separates the gut lumen from the epithelium. The integrity of the I. scapularis PM was essential for B. burgdorferi to efficiently colonize the gut epithelium. These data elucidate a functional link between the gut microbiota, STAT-signaling, and pathogen colonization in the context of the gut epithelial barrier of an arthropod vector.

Gα73B is a downstream effector of JAK/STAT signalling and a regulator of Rho1 in Drosophila haematopoiesis

JAK/STAT signalling regulates many essential developmental processes including cell proliferation and haematopoiesis, whereas its inappropriate activation is associated with the majority of myeloproliferative neoplasias and numerous cancers. Furthermore, high levels of JAK/STAT pathway signalling have also been associated with enhanced metastatic invasion by cancerous cells. Strikingly, gain-of-function mutations in the single Drosophila JAK homologue, Hopscotch, result in haemocyte neoplasia, inappropriate differentiation and the formation of melanised haemocyte-derived 'tumour' masses; phenotypes that are partly orthologous to human gain-of-function JAK2-associated pathologies. Here we show that Gα73B, a novel JAK/STAT pathway target gene, is necessary for JAK/STAT-mediated tumour formation in flies. In addition, although Gα73B does not affect haemocyte differentiation, it does regulate haemocyte morphology and motility under non-pathological conditions. We show that Gα73B is required for constitutive, but not injury-induced, activation of Rho1 and for the localisation of Rho1 into filopodia upon haemocyte activation. Consistent with these results, we also show that Rho1 interacts genetically with JAK/STAT signalling, and that wild-type levels of Rho1 are necessary for tumour formation. Our findings link JAK/STAT transcriptional outputs, Gα73B activity and Rho1-dependent cytoskeletal rearrangements and cell motility, therefore connecting a pathway associated with cancer with a marker indicative of invasiveness. As such, we suggest a mechanism by which JAK/STAT pathway signalling may promote metastasis.

Evidence for tissue-specific Jak/STAT target genes in Drosophila optic lobe development

The evolutionarily conserved JAK/STAT pathway plays important roles in development and disease processes in humans. Although the signaling process has been well established, we know relatively little about what the relevant target genes are that mediate JAK/STAT activation during development. Here, we have used genome-wide microarrays to identify JAK/STAT targets in the optic lobes of the Drosophila brain and identified 47 genes that are positively regulated by JAK/STAT. About two-thirds of the genes encode proteins that have orthologs in humans. The STAT targets in the optic lobe appear to be different from the targets identified in other tissues, suggesting that JAK/STAT signaling may regulate different target genes in a tissue-specific manner. Functional analysis of Nop56, a cell-autonomous STAT target, revealed an essential role for this gene in the growth and proliferation of neuroepithelial stem cells in the optic lobe and an inhibitory role in lamina neurogenesis.

Control of tissue morphology by Fasciclin III-mediated intercellular adhesion

Morphogenesis is dependent on the orchestration of multiple developmental processes to generate mature functional organs. However, the signalling pathways that coordinate morphogenesis and the mechanisms that translate these signals into tissue shape changes are not well understood. Here, we demonstrate that changes in intercellular adhesion mediated by the transmembrane protein Fasciclin III (FasIII) represent a key mediator of morphogenesis. Using the embryonic Drosophila hindgut as an in vivo model for organogenesis, we show that the tightening of hindgut curvature that normally occurs between embryonic stage 12 and 15 to generate the characteristic shepherd’s crook shape is dependent on localised JAK/STAT pathway activation. This localised pathway activity drives the expression of FasIII leading to its subcellular lateralisation at a stage before formation of septate junctions. Additionally, we show that JAK/STAT- and FasIII-dependent morphogenesis also regulates folds within the third instar wing imaginal disc. We show that FasIII forms homophilic intercellular interactions that promote intercellular adhesion in vivo and in cultured cells. To explore these findings, we have developed a mathematical model of the developing hindgut, based on the differential interfacial tension hypothesis (DITH) linking intercellular adhesion and localised surface tension. Our model suggests that increased intercellular adhesion provided by FasIII can be sufficient to drive the tightening of tube curvature observed. Taken together, these results identify a conserved molecular mechanism that directly links JAK/STAT pathway signalling to intercellular adhesion and that sculpts both tubular and planar epithelial shape.

JAK-STAT pathway in Drosophila morphogenesis: From organ selector to cell behavior regulator

One of the main contributions of Drosophila to the JAK-STAT field is the study of morphogenesis. JAK-STAT signaling controls the formation of many different structures through surprisingly different morphogenetic behaviors that include induction of cell rearrangements, invagination, folding of tissues, modulation of cell shape, and migration. This variability may be explained by the many transcription factors and signaling molecules STAT regulates at early stages of development. But is STAT just acting as an upstream inducer of morphogenesis or does it have a more direct role in controlling cell behaviors? Here we review what is known about how the canonical phosphorylation of STAT contributes to shaping the embryonic and imaginal structures.

The Drosophila JAK-STAT pathway in blood cell formation and immunity

Genetic alterations affecting the JAK-STAT signaling pathway are linked to several malignancies and hematological disorders in humans. Despite being one of the most extensively studied pathways, there remain many gaps to fill. JAK-STAT components are widely conserved during evolution. Here, we review the known roles of the JAK-STAT pathway in Drosophila immunity: controlling the different steps of hematopoiesis, both under physiological conditions and in response to immune challenge, and contributing to antiviral responses. We then summarize what is currently known about JAK-STAT signaling in renewal of the adult intestine, under physiological conditions or in response to ingestion of pathogenic bacteria.

A short receptor downregulates JAK/STAT signalling to control the Drosophila cellular immune response

Regulation of JAK/STAT signalling by a short, nonsignalling receptor in Drosophila modulates response to specific immune challenges such as parasitoid infestations.

The posterior signalling centre (PSC), a small group of specialised cells, controls hemocyte (blood cell) homeostasis in the Drosophila larval hematopoietic organ, the lymph gland. This role of the PSC is very reminiscent of the “niche,” the micro-environment of hematopoietic stem cells in vertebrates. We have recently shown that the PSC acts in a non–cell-autonomous manner to maintain janus tyrosine kinase/signal transducers and activators of transcription (JAK/STAT) signalling in hematopoietic progenitors (prohemocytes), thereby preserving the multipotent character necessary for their differentiation into lamellocytes, a cryptic and dedicated immune cell type required to fight specific immune threats such as wasp parasitism. In this report, on the basis of a knock out generated by homologous recombination, we show that a short type I cytokine-related receptor CG14225/Latran is required for switching off JAK/STAT signalling in prohemocytes. This is a prerequisite to massive differentiation of lamellocytes upon wasp parasitisation. In vivo and cell culture assays indicate that Latran forms heteromers with Domeless, the Drosophila type I cytokine signalling receptor related to mammalian GP130, and antagonises Domeless activity in a dose-dependent manner. Our analysis further shows that a primary immune response to wasp parasitism is a strong decrease in cytokine mRNA levels in the lymph gland, followed by an increase in the latran/domeless ratio. We propose that this sequence of events culminates in the complete inhibition of residual JAK/STAT signalling by Latran. JAK/STAT activity has been associated with several human diseases including leukaemia while knock-out studies in mice point to a central role of this pathway in hematopoiesis and regulation of immune functions. The specific function of Drosophila Latran is, to our knowledge, the first in vivo example of a role for a nonsignalling receptor in controlling a dedicated immune response, and thus raises the question of whether short, nonsignalling receptors also control specific aspects of vertebrate cellular immunity.

A specific microenvironment termed the “niche” supports long term maintenance of hematopoietic stem cells in vertebrates. A small group of specialised cells called the posterior signalling center (PSC) controls hemocyte (blood cell) homeostasis in the Drosophila larval hematopoietic tissue and thus fulfills a similar function to the vertebrate niche. The PSC acts at a distance to maintain JAK/STAT signalling in hematopoietic progenitors (prohemocytes), thereby ensuring their multipotent character. We report here that a short cytokine receptor encoded by CG14225/latran is required to extinguish JAK/STAT signalling in prohemocytes and thereby ensures their mass differentiation into lamellocytes, an immune cell type required to fight specific threats such as wasp parasitism. Domeless, a related receptor in Drosophila, was previously the only known cytokine receptor that signals through the JAK/STAT pathway. We show that Latran lacks the intracellular domains required for signal transduction and acts instead by antagonizing the function of Domeless in a dose-dependent manner. The role of Drosophila Latran in the repression of JAK/STAT signalling under specific immune conditions raises the question of whether short, nonsignalling receptors that antagonize full-length receptors could also control specific aspects of vertebrate immunity.

Transcriptional targets of Drosophila JAK/STAT pathway signalling as effectors of haematopoietic tumour formation

Although many signal transduction pathways have been implicated in the development of human disease, the identification of pathway targets and the biological processes that mediate disease progression remains challenging. One such disease-related pathway is the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) cascade whose constitutive misactivation by the JAK2 V617F mutation underlies most human myeloproliferative disorders. Here, we use transcript profiling of Drosophila haemocyte-like cells to identify JAK/STAT target genes, combined with an in vivo model for JAK-induced blood cell overproliferation, to identify the main effectors required for haematopoietic tumour development. The identified human homologues of the Drosophila effectors were tested for potential V617F-mediated transcriptional regulation in human HeLa cells and compared with small interfering RNA-derived data, quantify their role in regulating the proliferation of cancer-derived cell lines. Such an inter-species approach is an effective way to identify factors with conserved functions that might be central to human disease.

An efficient approach to isolate STAT regulated enhancers uncovers STAT92E fundamental role in Drosophila tracheal development

The ventral veinless (vvl) and trachealess (trh) genes are determinants of the Drosophila trachea. Early in development both genes are independently activated in the tracheal primordia by signals that are ill defined. Mutants blocking JAK/STAT signaling at any level do not form a tracheal tree suggesting that STAT92E may be an upstream transcriptional activator of the early trachea determinants. To test this hypothesis we have searched for STAT92E responsive enhancers activating the expression of vvl and trh in the tracheal primordia. We show that STAT92E regulated enhancers can be rapidly and efficiently isolated by focusing the analysis on genomic regions with clusters of putative STAT binding sites where at least some of them are phylogenetically conserved. Detailed analysis of a vvl early tracheal enhancer shows that non-conserved sites collaborate with conserved sites for enhancer activation. We find that STAT92E regulated enhancers can be located as far 60 kb from the promoters. Our results indicate that vvl and trh are independently activated by STAT92E which is the most important transcription factor required for trachea specification.

An evolutionary conserved function of the JAK-STAT pathway in anti-dengue defense

Here, we show that the major mosquito vector for dengue virus uses the JAK-STAT pathway to control virus infection. Dengue virus infection in Aedes aegypti mosquitoes activates the JAK-STAT immune signaling pathway. The mosquito's susceptibility to dengue virus infection increases when the JAK-STAT pathway is suppressed through RNAi depletion of its receptor Domeless (Dome) and the Janus kinase (Hop), whereas mosquitoes become more resistant to the virus when the negative regulator of the JAK-STAT pathway, PIAS, is silenced. The JAK-STAT pathway exerts its anti-dengue activity presumably through one or several STAT-regulated effectors. We have identified, and partially characterized, two JAK-STAT pathway-regulated and infection-responsive dengue virus restriction factors (DVRFs) that contain putative STAT-binding sites in their promoter regions. Our data suggest that the JAK-STAT pathway is part of the A. aegypti mosquito's anti-dengue defense and may act independently of the Toll pathway and the RNAi-mediated antiviral defenses.